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© Raymond Pettibon 


RESEARCH LIBRARY 
THE GETTY RESEARCH INSTITUTE 


JOHN MOORE ANDREAS COLOR CHEMISTRY LIBRARY FOUNDATION 





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PRACTICAL COLLOID 
CHEMISTRY 












PRACTICAL COLLOID 
CHEMISTRY 


WOLFGANG OSTWALD 


PROFESSOR OF THE UNIVERSITY OF LEIPZIG 


WITH THE COLLABORATION OF 


DR. P. WOLSKI and DR. A. KUHN 


TRANSLATED BY 


I. NEWTON KUGELMASS, M.D., Ph.D., Sc.D. 


YALE UNIVERSITY SCHOOL OF MEDICINE 
AND 


THEODORE K. CLEVELAND, Ph.D. 


WITH 22 ILLUSTRATIONS 


NEW YORK 
E. P. DUTTON AND COMPANY INC. 
PUBLISHERS 


THE GETTY RESEARCH 
INSTITUTE LIBRARY 





PREFACE TO FOURTH EDITION 


HIS book has been received so favourably that 
three editions were exhausted within two years. 
The author attributes this to the dire need for 
experimental knowledge of colloid chemistry after a period 
of theoretical interest in this science. If this be the 
explanation, it is all the more gratifying that this manual 
of experimental colloid chemistry has been welcomed to 
such an extent. 

The rapid sequence of the previous editions made it 
impossible to keep abreast of advances in colloid chemistry. 
Therefore, the present edition has been revised and re- 
edited in many respects. The experiments in this manual 
were performed and the procedures tested many times 
during the last four or five semesters by about two 
hundred students under the supervision of the author and 
Drs. P. Wolski and A. Kuhn. As a result, previous 
errors have been corrected and fifteen new experiments 
have been added, which include Chapter X on elementary 
dispersoid analysis. The author has also received 
suggestions from colleagues and invites such in the 
future. 

Some reviewers of the book suggested an index. The 
author purposely omitted it in previous editions because 
the manual contains a systematically arranged list of the 
experiments in the table of contents. If the manual is 
to give a survey of experimental colloid chemistry, it is 
preferable that the student learn to recognize systematic 


Vv 


vi PRACTIGAL COLLOID CHEMIST: 


colloid phenomena in conjunction with the experiments. 
Inspection of the manual readily reveals the chapter in 
which certain types of experiments are to be found. 
This manual is, of course, no reference work, but it does 
give a systematic presentation of the phenomena of 
colloid chemistry. 
WO. OSTWALD 

Leripzic May, 1924 


CONTENTS 


Peek eeaRoA LION OF COLLOIDAL 
SOLUTIONS 


A. CONDENSATION METHODS: 


OW, N Ho 


MAsTIc, PARAFFIN, SELENIUM SOLS . ; : 

RED GOLD SOL PREPARED WITH TANNIN 

RED GOLD SOL PREPARED WITH ALCOHOL . 

BLUE GOLD SOL PREPARED WITH HYDRAZINE 
HYDRATE : 

GOLD SOLS PREPARED WITH ILLUMINATING GAS 

GREEN GOLD SOLS PREPARED WITH ALCOHOL 

GOLD SOLS PREPARED WITH COMBUSTION GASES 


GOLD SOL PREPARED WITH A SOLID DISPERSION 
MEDIUM : , 


SILVER SOL PREPARED WITH TANNIN 
SILVER SOL PREPARED WITH HYDRAZINE HyDRATE 


. COLLOIDAL SULPHUR 

. ARSENIC TRISULPHIDE HyDROSOL 

. ANTIMONY TRISULPHIDE HyDROSOL 

. MERCURIC SULPHIDE HyYyDROSOL 

. MERCURIC SULPHIDE ALCOSOL . ; 

. SILVER [IODIDE HyDROSOL 

. SILVER CHLORIDE AND BROMIDE HYDROSOLS Z 
. PRUSSIAN BLUE HyYDROSOL 

. COPPER FERROCYANIDE HYDROSOL 

. FERRIC HYDROXIDE SOL 

. FERRIC HYDROXIDE SOL BY Hort DIALysis . 

. FERRIC HyDROXIDE SOL BY THE GRAHAM METHOD 


vill 


AP RW 


<i OO mE OS NOON OO WOON] NT Oy On Ol Orcs 


vill 


PRACTICAL COLLOID. CHEMIST. 


. ALUMINIUM HYDROXIDE SOL : < ; 2 
. MANGANESE PEROXIDE SOL 


S1ticic AcID SOL AND GEL : : é 


. SODIUM CHLORIDE SOL IN BENZENE 
. SODIUM CHLORIDE GEL ., 
. CALCIUM CARBONATE ALCOSOL 


B. DISPERSION METHODS: 


29. 
30. 
at. 
82. 
33: 
34: 
35: 
36. 
37: 
38. 
39. 
40. 
AUK 


42. 


43. 
44. 


45 


47: 


COLLOID FORMATION By MECHANICAL DISPERSION 
SILVER CHLORIDE HyDROSOL 

CADMIUM SULPHIDE HyYDROSOL . 

ALUMINIUM HYDROXIDE SOL , : : 
VANADIUM PENTOXIDE SOL PREPARED BY WASHING 


CONGO-RUBIN BLUE HypDROSOL; SOL FORMA- 
TION AND DISSOLUTION BY CHANGING THE 
HyYDRION CONCENTRATION 


MERCURIC SULPHIDE HybDROSOL BY WASHING 
AND PEPTIZATION . 4 : ’ : : 


FERRIC HYDROXIDE SOL; PEPTIZATIGN VEY stan 
ADDITION OF SOL-FORMING IONS ’ ; 


ALUMINIUM _. HYDROXIDE SOL By PEPTIZATION 
WITH HYDROCHLORIC ACID 


PRUSSIAN BLUE BY PEPTIZATION WITH OXALIC 
ACID 


STANNIC ACID By PEPTIZATION WITH AMMONIUM 
HYDROXIDE 


PREPARATION OF COLLOIDAL METALS AND METALLIC 
OXIDES BY ELECTRICAL DISPERSION 


PREPARATION OF COLLOIDAL LEAD BY ELECTRO- 
LVSISee : , : : : : ; 


LEAD PYROSOLS WITH SOLID LEAD CHLGRIDE AS 
A DISPERSION MEDIUM 


VON WEIMARN’S LAW 


STABILITY AND MOLECULAR SOLUBILITY OF COLLOID 
SYSTEMS : : : ; : : 


and’ 46. COLLOID FORMATION IN THE PRESENCE 
OF PROTECTIVE COLLOIDS 


PREPARATION OF PHOTOCHLORIDE SOLS : : 


PAGE 


16 


16 


16 


17 


18 


18 
19 


20 


20 
21 


ae 


CONTENTS 


Pe svIFRUSION, DIALYSIS, ULTRA- 


FILTRATION 


DIFFUSION : 


48. 


FUNDAMENTAL EXPERIMENTS ON GELATIN GELS 


49, 50 and 51. ANALYSIS OF POLYDISPERSE MIXTURES 


BY DIFFUSION 


DIALYSIS : 

52. SIMPLE DIALYSERS FOR PREPARATIVE PURPOSES 

53. FILTER DIALYSER 

54. SIMPLE DIALYSERS FOR COLLOID ANALYSIS 

55. DEMONSTRATION OF THE DIFFERENT DIALYSIS 
RATES oF DyYEs 

BEC ERALION - 

56. SEPARATION OF A POLYDISPERSOID BY FILTER 
PAPER .. 

ULEPRAHILTRATION : 

57. A SIMPLE ULTRAFILTER FOR COLLOID ANALYSIS 


58. 
59 


60. 
61. 
62. 


63: 


ILL 


PREPARATION OF SUCTION FILTERS 


ULTRAFILTRATION OF COLLOIDS HAVING VARIOUS 
DEGREES OF DISPERSION 


SEPARATION OF COLLOIDS AND MOLECULAR DIs- 
PERSE PHASES BY ULTRAFILTRATION 


SEPARATION OF DYE MIXTURES By ULTRAFILTRA- 
TION 


ULTRAFILTRATION OF A HETEROGENEOUS DISPERSE 
MIXTURE 


USE OF ULTRAFILTRATION FOR DETERMINING SMALL 
VARIATIONS IN DEGREE OF DISPERSION 


Sun AGr. EENSION; AND VISCOSITY 


Sone aACheTRNSION : 


64. 
65. 


SURFACE TENSION OF SOAP SOLUTIONS 


STALAGMOMETRIC STUDIES OF COLLOID CHEMICAL 
REACTIONS 


1X 


PAGE 


28 


28 
30 


31 
at 
32 
32 


32 


35: 


35 


x 


PRACTICAL COLLOID CHEMISiIiey 


B. VISCOSITY. VISCOSIMETRY EXPERIMENTS 


66. 
67. 


68. 


69. 
70. 
71, 


ON GELATIN: SOLUTIONS. 


INFLUENCE OF AGE OF SOLUTIONS UPON VISCOSITY 


INFLUENCE OF PRELIMINARY MECHANICAL TREAT- 
MENT UPON VISCOSITY 


INFLUENCE OF PRELIMINARY THERMAL TREATMENT 
UPON VISCOSITY 


INFLUENCE OF CONCENTRATION UPON VISCOSITY 
INFLUENCE OF TEMPERATURE UPON VISCOSITY 


INFLUENCE OF ADDITION OF ELECTROLYTES UPON 
VISCOSITY 


C. VISCOSIMETRY OF CHANGES OP ssi4 Fe 


72 
73- 
74. 


75: 
76. 


OF AGGREGATION : 


VISCOSIMETRY OF THE COAGULATION OF ALUMINIUM 
HYDROXIDE SOL 


VISCOSIMETRY OF THE SETTING OF PLASTER OF 
PARIS". 


VISCOSIMETRY OF THE FORMATION OF POTATO 
STARCH PASTE ‘ 

VISCOSIMETRY OF THE AGEING OF STARCH PASTE 

VISCOSIMETRY OF THE COAGULATION OF AN ALBU- 
MIN SOLUTION BY TEMPERATURE 


IV. OPTICAL PROPER ES. 


A. OPTICAL HETEROGENEITY FUREIOIi 


77: 


78. 


79. 
8o. 


81. 


82. 
83. 
84. 


85. 


DETECTION OF FAINT TURBIDITY BY MEANS OF 
THE FARADAY-ITYNDALL LIGHT CONE 


POLARIZATION OF THE TYNDALL LIGHT CONE 
TURBIDITY AND DEGREE OF DISPERSION 


CHANGES IN TURBIDITY OF AQUEOUS GELATIN 
WITH CONCENTRATION : 


EFFECT OF DEHYDRATION ON THE TURBIDITY 
OF SILIcIc ACID GELS 


GELATION AND TURBIDITY 
AGEING PHENOMENA AND TURBIDITY. 


INFLUENCE OF ELECTROLYTES ON THE TURBIDITY 
OF GELATIN GELS. 


CRITICAL TURBIDITY. 


PAGE 


39 
39 
40 
4I 
4I 


4l 


42 
43 


44 
47 


47 


52 
52 
53 


54 
55 
55 
56 


56 
56 


CONTENTS 


B. ULTRAMICROSCOPY : 


86. 
87. 
88. 


TYPICAL ULTRAMICROSCOPIC IMAGES . 
ULTRAMICROSCOPY OF GELATION 


ULTRAMICROSCOPY OF THE AGEING OF STARCH 
PASTES 


89. ULTRAMICROSCOPY DURING FLOCCULATION . ; 
ROTATION OF PLANE OF POLARIZED LIGHT 


go. 
ol. 


BY COLLOIDS 
OpTIcAL ROTATION BY GELATIN SOLUTIONS 


OPTICAL PROPERTIES OF A VANADIUM PENTOXIDE 
SOL 


COLOUR OF COLLOID SOLUTIONS. COLOUR 


92. 


OMe sCOLOURLESS COLLOIDS” 
OPALESCENT SOLUTIONS 


C. COLOURS OF COLLOIDAL METALS: 


93: 
94. 
95- 
96. 


97. 
08. 
99. 


Ioo. 
Iol. 


102. 
103. 


104. 


105. 


106. 


POLYCHROMISM OF GOLD SOLS 
POLYCHROMISM OF SILVER SOLS. 
POLYCHROMISM OF SULPHUR SOLS 


CoLOUR CHANGES IN GOLD SOLS DURING FLOCCU- 
ATION . 


CoLOUR CHANGES IN CONGO-RUBIN 
COLOUR AND DEGREE OF DISPERSION. 
ULTRAMICROSCOPIC COLOURS ¢ 5 


See LRICAL PROPERTIES 


POSITIVE AND NEGATIVE COLLOIDS . 


CHANGING THE CHARGE ON COLLOID PARTICLES 
BY VARYING THE MoDE OF PREPARATION 


POSITIVE AND NEGATIVE FERRIC HyDROXIDE SOLS 


INFLUENCE OF (H+) anp (OH~-) Ions ON THE 
ELECTRICAL CHARGE OF A SUSPENSOID . 


CHANGES IN THE ELECTRIC CHARGES OF FERRIC 
HYDROXIDE SOLS BY FILTRATION 


DETECTION OF ELECTRICALLY CHARGED COLLOID 
PARTICLES BY CAPILLARITY . 


CAPILLARITY WITH PREPARED FILTER PAPER 


Xi 


63° 


64 


66 


Xl 


107. 


PRACTICAL COLLOID CHEMIST 


VI. EXPERIMENTS WITH GEES 


MECHANICAL PROPERTIES OF PASTES P 


A, GELATION © 


to8. 


109. 


IIo. 


Tees 
Tee 
773: 


DETERMINATION OF GELATION CONCENTRATION 
AND LEIME F , ; ; ; : ‘ 


DETERMINATION OF SOLIDIFYING AND SOFTENING 
TEMPERATURES . ; . d , 4 


INFLUENCE OF PRELIMINARY THERMAL TREAT- 
MENT ON GELATION 


INFLUENCE OF ACIDS AND ALKALIES ON GELATION 
INFLUENCE OF SALTS ON GELATION. 
INFLUENCE OF NON-ELECTROLYTES ON GELATION 


Bao WEUCEING. 


114 


115. 


I16. 


LE. 
118. 


aT LOs 


120: 


WAIN 


I22. 


123\ 
124. 


QUALITATIVE DEMONSTRATION OF THE SWELLING 
PROCESS 


QUALITATIVE DEMONSTRATION OF SWELLING IN 
VAPOUR 


DEMONSTRATION OF HEAT OF SWELLING 
RATE OF SWELLING AND SWELLING MAXIMUM 


INFLUENCE OF ACIDS AND BASES ON THE SWELL- 
ING OF GELATIN OR FIBRIN . 


LocaLt AcID SWELLING. AN EXPERIMENT ON 
THE THEORY OF INSECT STINGS 


INFLUENCE OF SALTS UPON THE TURGESCIBILITY 
OF GELATIN 


INFLUENCE OF MIXTURES OF ACIDS, ALKALIES 
AND SALTS ON THE SWELLING OF GELATIN 


INFLUENCE OF NON-ELECTROLYTES ON THE SWELL- 
ING OF GELATIN 


SWELLING AND COLLOID FORMATION. 
SWELLING OF RUBBER IN VARIOUS LIQUIDS : 


Co SY NALRE SI Se 


125. 
126, 
1278 


SYNZRESIS OF GELATIN, AGAR AND STARCH GELS 
SYNZRESIS OF SILICIC ACID GELS . ; : 


SYNZRESIS OF A RUBBER GEL DURING VULCANIZA- 
fION . é é J d : 


PAGE 


88 


88 


gI 
gI 
92 
93 


94 


aS 


95 
100 


Iol 
102 
103 
103 


104 
104 
105 


106 
107 


108 


CONTENTS 


PRECIPITATION REACTIONS AND RELATED 
PHENOMENA IN GELS: 


128. LIESEGANG RINGS ; 

129. Forms oF METALLIC LEAD PRECIPITATES IN GELS 
130. Forms oF METAL SILICATE PRECIPITATES. 

131. ORIGIN OF NATIVE ALUMINA 

132. PRECIPITATE MEMBRANES 

133. Gas BUBBLES IN GELS 


DRYING AND FREEZING OF GELS: 


134. FIGURE FORMATION IN THE DRYING OF EGG 
WHITE 


135. THE DRYING OF GELATIN SOLUTIONS : <= 


136. ICE CRYSTALS IN GELATIN GELS 


Vila DSORPTION 


ADSORPTION AT THE INTERFACE LIQUID- 
SOLID: 


137. QUALITATIVE DEMONSTRATION OF ADSORPTION . 


138. PROOF OF THE PRESENCE OF ADSORBED DYES AT 
THE INTERFACES . 


139. SURFACE COLOURS OF ADSORBED DYES 


140. ADSORPTION OF LEAD NITRATE BY ANIMAL 
CHARCOAL, 


141. ADSORPTION OF ALKALOIDS BY ALUMINIUM SILI- 
CATE 


142. INFLUENCE OF DILUTION. REVERSIBILITY OF 
_ ADSORPTION 


143. QUANTITATIVE ADSORPTION OF ACETIC ACID 


144. ADSORPTION OF CRYSTAL PONCEAU AND METHY- 
; LENE BLUE BY WOOL . 


145. SPECIFIC DyE ADSORPTION By SILICIC ACID AND 
ALUMINIUM HyDROXIDE GELS 


ADSORPTION AT THE INTERFACE LIQUID. 
LIQUID : 


146. ADSORPTION OF COLLOIDAL COPPER SULPHIDE 
AT THE INTERFACE, WATER-CHLOROFORM 


Xlii 


PAGE 
108 


IIo 
Jee 
II2 


Lr 
II3 


114 
114 
I16 


118 


119 


PRACTICAL COLTOID CHEMIST. 


ISOSTABLE ALBUMIN SOL . 


PAGE 


128 
129 
129 


130 


130 


148 


149 
149 


150 


151 


X1V 
147. ADSORPTION OF GELATIN AT THE INTERFACE, 
WATER-BENZOL P 
148. ADSORPTION OF A COARSELY DISPERSE POWDER 
AT THE INTERFACE, LIQUID-LIQUID 
149. SEPARATION OF COARSELY DISPERSE MIXTURES 
BY SELECTIVE ADSORPTION 
150. FLOTATION OF PRINTED AND UNPRINTED PIECES 
OF PAPER : : : . 3 
C. ADSORPTION AT THE INTERFACE, LIOUID- 
GAS: 
151. PEPTONE MEMBRANES 
VIII. COAGULATION, PEPTIZATION AND 
RELATED PHENOMENA 
A. FLOCCULATION OF SUSPENSOIDS. 
152. QUALITATIVE DEMONSTRATION OF THE ELECTRO- 
LYTIC FLOCCULATION OF SUSPENSOIDS 
153 and 154. ELECTROLYTIC FPLOCCULATION — GF 
COPPER SULPHIDE HyDROSOL : ye er 
155. ELECTROLYTIC FLOCCULATION OF GOLD SOL 
156. ELECTROLYTIC FLOCCULATION OF CONGO-RUBIN 
157. FLOCCULATION OF FERRIC-HYDROXIDE SOL 
158. ‘“ ABNORMAL SERIES ’”’ wiTH MAsTIC SOL . 
159. INFLUENCE OF TEMPERATURE ON THE FLOCCULA- 
TION OF CONGO-RUBIN . 

REVERSIBILITY OF SUSPENSOID FLOCCULATION : 
160. FLOCCULATION OF SUSPENSOIDS BY DIALYSIS 
161. FLOCCULATION BY AN ELECTRIC CURRENT 

B. FLOCCULATION OF EMULSOIDS: 
162. QUALITATIVE DEMONSTRATION OF SUSPENSOID 
AND EMULSOID FLOCCULATION 
163. ACID AND ALKALI FLOCCULATION OF CASEIN SOL. 
ISOLABILE ALBUMIN SOLS ‘ 
164. NEUTRAL SALT FLOCCULATION OF HA&:MOGLOBIN. 


152 


165. 
166. 
167. 
168. 
169. 
170. 


ts 


172. 


CONTENTS 


“ABNORMAL SERIES’”’ WITH DIALYSED EGG 
WHITE 


INFLUENCE OF TEMPERATURE ON THE ELECTRO- 
LYTIC FLOCCULATION OF GELATIN SOLUTIONS 


FLOCCULATION OF HYDRATED GLOBULIN BY 
ELECTROLYTIC EXTRACTION . 


REVERSIBLE AND IRREVERSIBLE ELECTROLYTIC 
FLOCCULATION OF EGG WHITE 


FLOCCULATION OF H#&MOGLOBIN By ALCOHOL 
COAGULATION OF DIALYSED EGG WHITE BY HEAT 


INFLUENCE OF ELECTROLYTES ON THE COAGULA- 
TION TEMPERATURE OF DIALYSED EGG WHITE 


THEORY OF EMULSOID PRECIPITATION 


fee PUA INTERACTION: OF COLLOIDAL 
SULUTIONS : 


Pee ee LATION OF TWO COLLOIDS: 


173. 


174. 


175. 


RECIPROCAL FLOCCULATION OF ARSENIC TRI- 
SULPHIDE AND FERRIC-HYDROXIDE SOLS 


RECIPROCAL FLOCCULATION OF CONGO-RED AND 
NIGHT-BLUE 


RECIPROCAL TITRATION OF TWO DYES 


Mee PROTECTIVE ACTION : 


176. 
a. 
178. 
179. 


GoLp NUMBERS 
RuBIN NUMBERS 
PURPLE OF CASSIUS 
‘“ RUBIN PURPLE”’’ , 


Perea er liZATION AND DISSOLUTION : 


18o. 
TST. 


182. 


L353. 


PEPTIZATION PHENOMENA ‘ p ; 

DISSOLUTION OF A RED GOLD SOL BY POTASSIUM 
CYANIDE ; 2 a s 

BEHAVIOUR OF SILVER SOLS TOWARD NITRIC 
ACID : : : 

COAGULATION AND DISSOLUTION OF SILVER BrRo- 
MIDE BY AMMONIA 


XV 


PAGE 


156 
156 
157 


157 
158 


159 


159 
160 


162 


163 
163 


165 
166 
167 
167 


168 


169 


169 


169 


XV1 


PRACTICAL COLLOID CHEMiza i 


IX. COMMERCIAL COLLOIDS ANDO Tims 
MATERIAL FOR DEMONSTRATION EXPERI- 


MENTS 


A. INORGANIC COMMERCIAL COLLOIDS: 


le 


COLLOIDAL METALS 
COLLOIDAL COMPOUNDS . 


INORGANIC COLLOIDS WITH SOLID #£ DISPERSION 
MEDIA 


ORGANIG COMMERCIAL COULDE@IS: 
I. SERIES OF DISPERSOIDS OF VARYING DEGREES OF 
DISPERSION 


2. DISPERSE SYSTEMS ACCORDING TO THEIR PHYSICAL 
STATE 


TABLE OF DISPERSE SYSTEMS. 


xX. DISPERSOID ANALYSIS 


A. GENERAL FIXATION OF DEGREES OF Dis- 


B. 


PERSION 
SPECIAL COLLOID - ANAL 


TABLE OF NORMAL SOLUTIONS 


PAGE 
Ton 
177 


I81 


183 


184 
186 


187 
188 


190 


PRACTICAL COLLOID 
CHEMISTRY 


I 


PREPARATION OF COLLOIDAL 
SOLUTIONS! 


A. CONDENSATION METHODS 
(_ J ectoiat str methods for the preparation of 


colloidal solutions are applicable to molecular 

disperse systems. Certain molecules of such a 
solution are brought into the colloid state by coalescence. 
This is obtained by stabilizing the precipitating particles 
so that they will remain dispersed within the range of 
colloid dimensions.’ 


BONDENSATION BY DECREASE IN SOLUBILITY 


Expt. 1. Add drop by drop with continuous stirring 
5 to 10 c.c. of a 3 per cent. alcoholic solution of mastic or 
colophony to 100 c.c. of distilled water. The resulting 
milky white, strongly opalescent sol may be freed from 
the coarser particles by filtration. The alcohol may be 
removed by heating the colloidal solution which inci- 


1 An excellent presentation of the methods of preparation is 
given by The Svedberg, Preparation of Colloidal Solutions of 
Inorganic Substances, 3rd edition, Dresden, 1922. 

I 1 


D PRACTICAL COLLOID CHERES it: 


dentally produces a partial coagulation of the sol. Un- 
heated sols remain stable for years. 

A paraffin hydrosol may be prepared in a similar 
way from a dilute alcohol solution of solid or liquid 
paraffin, i.e. a 2-3 per cent. solution, otherwise most of 
the particles are coarsely disperse. 

A selenium sol (A. Gutbier) is obtained by dissolving 
precipitated selenium in concentrated hydrazine hydrate 
and pouring a few drops of this solution into distilled 
water. The resulting sol is red and opalescent. 

A sulphur sol (P. P. von Weimarn and B. Malytschew) 
is prepared by adding a saturated solution of sulphur in 
absolute alcohol to a large volume of distilled water. 

Colloidal emulsions as castor oil, petroleum, essential 
oils such as oil of rosemary and cassia are all similarly 
prepared by precipitation from dilute alcohol solutions. 


CONDENSATION BY CHEMICAL PRECIPITATION 


Colloidal gold—One per cent aqueous solutions of 
HAuCl,.4H,O or AuCl, are used throughout these ex- 
periments. Each solution is made neutral or very slightly 
alkaline to litmus by cautious addition of pure Na.COs. 
Excess of hydroxions as well as impure carbonate decom- 
pose the solution. If the solution is to be preserved for 
further use, neutralize and dilute 100 times (0-01 per cent.) 
the desired amount of the I per cent solution previous to 
performing the experiment. 

Expt. 2. Red gold sol prepared with tannin— 
Heat to boiling 100 c.c. of distilled water and 5-10 c.c of 
the o-or per cent. gold solution in an Erlenmeyer flask. 
To this solution add, drop by drop, a freshly prepared 
I per cent. aqueous tannin solution. Stir intermittently 
for 30 seconds and continue adding the tannin solution 
until an intense red coloration is obtained. Appreciable 


PREPARATION OF COLLOIDAL SOLUTIONS 3 


amounts of gold chloride and tannin may be added before 
coarsely disperse blue or grey green metallic depositions 
form. Only fresh tannin solutions must be used because 
old solutions spoil despite the addition of preserva- 
tives. 

This experiment is especially suited for demonstration, 
because the sol is usually non-sensitive to the impurities of 
distilled water in the presence of tannin. Gold may be 
preserved for years with chloroform and toluol without 
any more than a slight browning. A sensitive sol may, 
however, be prepared by avoiding an excess of tannin 
(J. Matula). Such a sol is used to demonstrate the 
phenomenon of colour transition from red to blue as will 
be described below. 

Expt. 3. Red gold sol prepared with alcohol— 
Heat to boiling 100 c.c. of distilled water and 5 to Io c.c. 
of 0-or per cent. gold solution. Remove the burner, and 
add proportionately 5 to Io c.c. of concentrated alcohol 
with constant stirring. Continue heating for 15 to 20 
minutes until the distinct cherry-red colour appears. 
More gold solution and alcohol may be added to this sol 
without any marked change in its red coloration. The 
characteristic red colour of the sol persists even on strong 
evaporation, but is immediately transformed to blue 
violet upon addition of electrolytes. This gold sol is 
strongly sensitive to electrolytes in marked contrast to 
the tannin gold sol. 

Expt. 4. Blue gold sol prepared with hydrazine 
hydrate (A. Gutbier)—To a mixture of 100 c.c.. of water 
and about 10 c.c. of a 0-01 per cent. unneutralized gold 
solution, add a few drops of a diluted hydrazine hydrate 


10On the preparation of red gold sols with formaldehyde, see 
R. Zsigmondy, Liebigs Ann., 301, 30 (1898 ); Z. phys. Chem., 56, 
65 (1906); H. Morawitz, Kolloidchem. Bethefte, 1, 324 (1910) ; 
The Svedberg, Joc. cit. 


4 PRACTICAL COLEOID CHERI aia 


solution (I c.c. of the C.P. 50 per cent. solution diluted 
with a litre of water). 

Phenylhydrazinechlorhydrate and  hydroxylamine- 
hydrochloride may be used in the same concentration to 
replace hydrazinehydrate. 

Expt. 5. Gold sol prepared with illuminating 
gas—Gold sols may be obtained by the continuous 
passage of coal gas for about 24 hours into 100 c.c. of 
O-or per cent. neutralized gold solution. The colours of 
the sols, red, violet or blue, depend upon the carbon 
monoxide concentration of the illuminating gas. Carbon 
monoxide, prepared by heating oxalic and sulphuric 
acids and washing the gas, yields sols of a distinct red 
coloration especially if distilled water or conductivity 
water has been used (J. Donau). 

Expt. 6. Green gold sol prepared with alcohol— 
Follow the procedure in Expt. 3 and use concentrated 
gold solutions ; 100 c.c. of a 0-01 per cent. solution and 
5 to 10 c.c. of alcohol. The blue-green sols obtained are 
coarsely disperse and unstable. 

Expt. 7. Gold sols prepared with combustion 
gases—Direct a hydrogen flame (J. Donau) or more 
simply a small Bunsen flame? obliquely across the surface 
of a gold solution (10 to 20 c.c. of 0-01 per cent. gold 
solution and 100 c.c. of water) contained in a porcelain 
dish. Stir the solution continuously by means of a glass 
rod. A red to a red-violet sol is obtained after 5 to 
ro minutes. Further reduction is obtained by holding 
the burner at a distance and is manifested by a deepening 
of the colour. 

Expt. 8. Gold sol prepared with a solid dispersion 
medium (J. Donau)—-Heat some borax contained in a 
platinum loop, dip it into a o-o1 per cent. (or more diluted) 


1 A yery luminous Bunsen flame will contaminate the solution 
with soot particles. . 


PREPARATION OF COLLOIDAL SOLUTIONS 5 


gold solution and heat again until the formation of a 
clear molten mass. Beads are obtained whose colours 
depend upon the quantity of gold and the duration of 
heating. Some of the beads are ruby-red and trans- 
parent, while others appear dark blue in transmitted 
light and yellow-brown in reflected light. 

Coloured sols are also obtained by fusing microcosmic 
salt with gold chloride in a porcelain crucible and then 
pouring the molten liquor drop by drop upon a glass 
plate. With increasing amounts of gold chloride one 
obtains pearls that appear ruby-red to yellow in reflected 
light and blue in transmitted light. 


Polo AL SILVER 


Expt. 9. Silver sol prepared with tannin—Add 
a few drops each of freshly prepared I per cent. tannin 
solution and Io per cent. sodium carbonate solution to 
about 100 c.c. of a 0-00IN solution of AgNO;. Upon 
warming, an intense red to yellow-brown sol is obtained, 
which is not very sensitive to electrolytes. 

Expt. 10. Silver sol prepared with hydrazine 
hydrate (A. Gutbier)—Add a few drops of the diluted 
ihydrazine hydrate solution (r c.c. of C.P. 50 per cent. 
solution diluted in a litre of water) to 100 c.c. of a solution 
containing I c.c. of o-IN AgNO, in a litre of water. Upon 
heating, a sol is obtained which is bright yellow in trans- 
mitted light and green in reflected light. Silver sols of 
other colours may be prepared according to Expt. 94. 


COLLOIDAL SULPHUR (M. RAFFO) 
Expt. 11. Dissolve 50 g. of crystalline sodium thio- 
sulphate (Na.S.03.5H.O) in 30 c.c. of water. Weigh 
70 g. of concentrated H,SO, in a 300 c.c. beaker and keep 


6 PRACTICAL COLLOID CHEMISTRY 


on ice. Add the thiosulphate solution, drop by drop, 
preferably from a dropping funnel into the sulphuric 
acid. This yields a thick yellow mass. Add further 
30 to 50 c.c. of water, warm on the water-bath for ro 
to 15 minutes to drive off the formed -SO,. The mass 
now becomes a Clear yellow liquid. After cooling, filter 
over glass wool or through a coarse towel, yielding a | 
concentrated colloidal milk of sulphur contaminated with 
Na.SO,. The sol becomes clear on warming and again 
cloudy on cooling. Such sols containing very small 
amounts of sulphate may be kept for a long time without 
any appreciable decomposition. Purification of the sol 
by dialysis is possible to only a limited extent, since 
small quantities of Na,SO, are indispensable for its 
stability. 

A sulphur sol can be prepared more simply by neutral- 
izing with hydrochloric acid a very dilute solution of 
ammonium sulphide, yielding a distinctly opalescent 
solution. 

Sulphur sols of different degrees of dispersion may be 
prepared according to the method of S. Odén [Koll. 
Zetischy. 8, 196 (IOIT)* °9, 100s torn 


COLLOIDAL SULPHIDES 


Expt. 12. Arsenic trisulphide hydrosol (H. 
Schulze)—Prepare a saturated solution of arsenious acid 
by boiling As.O; in water. Upon cooling to room 
temperature the strength of the solution is about 2 per 
cent. Dilute 50 c.c. of this solution to 200 c.c. to give a 
0-5 per cent. solution. Pass in a stream of H,5 for about 
five minutes. An intense yellow to yellow-red sol is 
obtained which turns green-yellow on dilution. Con- 
centrated sols may be prepared by further addition of 
the arsenious acid to the already prepared sol and may 


fre nnelON OF COLLOIDAL SOLUTIONS 7 


again be precipitated from it. Coarsely disperse aggre- 
gates that may form should be filtered or separated by 
dialysis. 

Expt. 13. Antimony trisulphide hydrosol (H. 
Schulze)—An intense yellow-red sol is obtained directly 
by bubbling H.S into a solution of 0-2 to 0:3 g. of tartar 
emetic (potassium antimonyl tartrate). The H,S must 
not be passed in for too long a time else slight coalescence 
results. The sol may be purified by dialysis. 

Expt. 14. Mercuric sulphide hydrosol (A. Lotter- 
moser)—Pass H.S into a saturated solution of Hg(CN), 
or into a diluted solution (a few drops of the concen- 
trated to 200 c.c. of water). A deep brown sol is obtained 
without any aggregates. | 

Expt. 15. Mercuric sulphide alcosol (A. Lotter- 
moser)—Pass H.S into a saturated solution of Hg(CN), 
in 97 per cent. alcohol and a dark brown alcosol results 
which, on inspection, appears like the hydrosol. 


Peet DE eAND] CYANIDE HYDROSOLS 

Expt. 16. Silver iodide hydrosol (A. Lottermoser) 
—Add 2 c.c. of o 1N AgNO, to a dilute KI solution 
(2 to 3 c.c. 0-IN KI in 100 c.c. of water) and a green- 
yellow milky sol results. Most of the KI may be removed 
by dialysis, although the sol is quite stable for months 
without any dialysis. 

Concentrated but less stable sols may be prepared by 
adding 7 to 8 c.c. of o-IN AgNO; to 20 c.c. of 0-05N KI, 
or by adding 5 to 6 c.c. of o-IN KI to about 20 c.c. of 
0:05N AgNO;. The first sol is more stable than the 
second. The proportions as given are essential for 
making good sols. See A. Lottermoser, Zeittschr. f. 
physik. Chemie 62, 370 (1908); Journ. f. prakt. Chemie 
73, 374 (1906). The electrical charges of these sols are 
as given in Experiment Ior. 


8 PRACTICAL COLLOID CHEATS ic 


Expt. 17. Silver chloride and bromide hydrosols 
—Add about 2 to 3 c.c. of o-IN solution of KCl or KBr 
to 100 c.c. of water with stirring and then add 1 to 2 c.c. 
of o-IN AgNO,;. <A milk-white silver chloride sol and a 
grey silver bromide hydrosol result. Cf. Expt. 44 for 
the stability of these sols. 

Expt. 18. Prussian blue hydrosol—Add a freshly 
prepared, diluted solution of FeCl, (without warming) 
to a diluted solution of potassium ferrocyanide (0-1 gr. 
per litre). The amount of FeCl, added must be less than 
that required by the molecular proportion, FeCl,: 
Khel GN) qe 

Concentrated stable hydrosols may be prepared by 
mixing concentrated solutions of FeCl, and potassium 
ferrocyanide. The ferric chloride solution is added drop 
by drop with constant stirring until a thick paste results. 
Sols of any concentration are obtained by adding the 
paste to water. 

Expt. 19. Copper ferrocyanide hydrosol—Add 1 
per cent. CaSO, or CaCl, to 100 Cic) Of (iend tert 
(CN), used in Expt. 18. A clear brown-red stable sol 
is obtained. 


HYDROXIDE AND OXIDES YDEO-Or. 


Expt. 20. Ferric hydroxide sol (Krecke)—Heat to 
boiling about 200 c.c. of distilled water. Add about 
20 c.c. of freshly prepared, yellow 2 per scemiseisets 
solution and an intense red-brown sol is obtained after a 
few minutes. Since this hydrosol is reversible on cooling, 
it is best to dialyse it while warm. 

Expt. 21. Ferric hydroxide sol by hot dialysis 
—Prepare a collodion dialysing sac according to Expt. 
54. Suspend it in a beaker of water and fill the sac 
with o-or M. FeCl;. Place the arrangement on a water- 


PREPARATION OF COLLOIDAL SOLUTIONS: g 


bath. Change the water contained in the beaker until 
the test for chloride is negative. Quantities of hydrosol 
up to 50 c.c. are dialysed completely within two days 
provided the arrangement is kept in a warm place 
overnight. Salt solutions of concentration greater 
than o-or M. coalesce. 

Expt. 22. Ferric hydroxide sol (Thomas Graham) 
—To a half saturated solution of FeCl, add gradually a 
2N (NH,).CO; solution until the resulting precipitate 
continues to dissolve on stirring. Dialyse preferably by 
warming. 

See Expts. 36 and i102 for the dispersion method of 
preparing concentrated Fe(OH), sols. 

Expt. 23. Aluminium hydroxide sol—Dilute a 
Io per cent. solution of aluminium acetate twenty times 
and warm on a water-bath until the odour of acetic acid 
disappears. The resulting Al(OH), sol is clear and 
colourless. The sol is easily detected and its concen- 
tration determined by precipitating it with aqueous 
K ,Fe(CN).. 

See Expts. 32 and 37 for dispersion methods of preparing 
concentrated sols. 

Expt. 24. Manganese peroxide sol (J. Cuy)— 
Warm a 0:05N solution of KMnO, to boiling and add 
gradually concentrated NH,OH until the solution is 
coffee coloured. Boil until the last traces of ammonia 
have been expelled. The KOH formed in the reaction 
does not interfere with the sol and need not be neutralized. 
The sol is clear, transparent, brown and stable when 
unexposed to air. It is easily coalesced by contact with 
filter or parchment paper. Sols incompletely reduced to 
MnO, are unstable. They appear red due to the presence 
of undecomposed KMnO, and coalesce upon filtration. 
The supernatant liquid is also red. Addition of alcohol 
colours it yellow due to reduction of KMnO,. The sol 


10 PRACTICAL COLLOID CHEMISTS 


may be maintained stable by addition of small amounts 
of alcohol. 

Expt. 25. Silicic acid sol and gel—Add a 5 per 
cent. solution of water glass ! to 100 c.c. of o-IN HCl and 
stir. The resulting sol must be acid to avoid spontaneous 
gelation.2 If acid, the solution will be colourless to 
phenolphthalein. Colloidal SiO, is detected by precipita- 
tion with ammoniacal cupric oxide or Ba(OH),. 

Silicic acid gel is prepared by adding a Io per cent. 
water glass solution to an equal volume of 2N HCl. The 
gel first appears clear and then opalescent. 


COLLOIDAL ALKALI AND ALKALI EARTH SALTS 


Expt. 26. Sodium chloride sol in benzol (C. Paal) 
—Sodium malonic ester and ethyl chloracetate react on 
warming to form NaCl and ethyl ethylene tricarboxylate. 
Prepare a benzene solution of sodium malonic ester as 
follows: Add 5 g. of malonic ester (CH.[CO.C.H;].) to 
30 g. of benzol (previously dried over sodium) and then 
add o-7 g. of sodium wire. Transfer the solution to a 
flask fitted with a reflux condenser and warm on a water- 
bath until the sodium is completely dissolved. Allow 
the solution to cool, add 4:0 g. of chloracetic acid ester 
(CICH,CO.C.H;) and again warm with continuous 
stirring. With dry benzene the resulting sol is usually 
yellow and strongly opalescent. The yellow, highly 


1 The concentration of commercial water glass varies consider- 
ably. The Merck pharmaceutical preparation is a Io per cent. 
solution. The concentrations of sodium silicate solutions referred 
to in the text are based on ash determinations. 

2 A preliminary experiment is advisable in order to determine 
how many c.c. of water glass are necessary for instantaneous 
solidification and- one-third to one-half of this amount is used 
in the experiment (R. Zsigmondy). 

3 At times an opalescent rigid gel is formed. 


PeeenineablON OF COLLOIDAL SOLUTIONS | ir 


disperse sol becomes white and coarse with evident 
sedimentation when the benzene used contains water or 
when the sol is allowed to stand. The yellow sol may be 
kept stable for years in a well-stoppered container. 

Expt. 27. Sodium chloride gel (L. Karczag)—Dry 
carefully 15 to 20 g. of sodium salicylate in order that 
the gel to be prepared may be obtained without difficulty. 
Pour 20 g. of thionyl chloride (SOCI,) into a glass-stop- 
pered vessel under a hood and add to it 15 to 20 g. of the 
dried sodium salicylate. The flask is kept loosely 
stoppered to permit the escape of SO, formed in the 
reaction. After a short time the lower layers of the 
salicylate appear to be gradually transformed into a 
- green-orange opalescent gel. The process is complete 
within a day and the resulting gel is of the consistency of 
soft soap. The gel, kept sealed up, remains stable for 
several weeks. 

Expt. 28. Calcium carbonate alcosol (C. Neuberg) 
—-Heat lime (CaO) to glowing, pulverize and add 2 to 3 
er. to 50 c.c. of freshly distilled absolute methyl alcohol. 
Pass CO, into this solution for several hours. The CaO 
is thereby completely dissolved. Decant the super- 
natant blue-yellow opalescent organosol of CaCO;. The 
milky sol is transformed into a gel in the presence of 
excessive CaO and on prolonged standing in the cold. 
This alcosol remains stable for weeks in sealed containers. 


B. DISPERSION METHODS 


Dispersion methods for the preparation of colloidal 
solutions involve the continuous division of non-disperse 
or coarsely disperse substances into the colloidal state. 


1 The author has a preparation that has remained of a milky 
consistence since IgIo, 


12 PRACTICAL COLLOID CHEMTSt i 


COLLOID FORMATION BY MECHANICAL DISPERSION 


Expt. 29. A suspension of potato starch in cold . 
distilled water is decomposed by a baryta filter so that 
no iodine reaction is obtained for the filtrate. On the 
other hand, some moist starch ground in an agate mortar 
yields a starch hydrosol, most of which passes through 
the filter and gives a positive iodine reaction for the 
filtrate (G. Wegelin). 

Starch granules, according to a private communication 
from H. Luers and C. Lintner, may contain occluded 
substances which disperse spontaneously on mere standing 
of starch in water. 


CHEMICAL DISPERSION METHODS 


Chemical dispersion methods involve the use of freshly 
prepared precipitates which are transformed into the 
colloid state. These precipitates usually contain ad- 
sorbed electrolytes which have a coagulating effect on 
colloids and hence retard colloid formation ; for example, 
H,SO, in the precipitation of CdS by bubbling H,S 
into CdSO,. Electrolytes so formed must be partially 
washed from the precipitate because an optimum con-: 
centration of electrolyte is indispensable in the formation 
of stable hydrosols. Precipitation ordinarily occludes 
too great a concentration of “‘ sol-formingions.”’ Washing 
with water until optimum ionic concentration of the 
precipitate is attained yields a stable hydrosol provided 
that coagulating ions are simultaneously removed (Expts. 
30-34). The precipitate may first be washed completely 
or decanted free from the ions which retard colloid 
formation and then a definite concentration of electrolyte 
added to obtain the desired hydrosol. This process is 
known as peptization (Expts. 35-39). 


Pieter ten Or COLLOIDAL SOLUTIONS | 13 


A. CHEMICAL DISPERSION METHODS BY WASHING 


Expt. 30. Silver chloride hydrosol—To 20 c.c. of 
o-oIN AgNO, add more than the equivalent amount 
of 2N NH,Cl, i.e. about 15 drops and filter. The last 
portions of the filtrate are clear. Wash the precipitate 
with water until the filtrate becomes clear. The milky 
portions obtained consisted of the AgCl hydrosol. This 
indicates that the optimum ionic concentration. has been 
exceeded. Good results are obtained with the concen- 
tration given. 

Expt. 31. Cadmium sulphide hydrosol—To a 
5 to 10 per cent. solution of CdSO, add concentrated 
NH,OH until the resulting precipitate continues to dis- 
solve. Pass in H.S for about 5 minutes. A heavy 
precipitate and a yellow coloured supernatant liquid 
results on standing. Wash by decantation until the 
precipitate settles more slowly; if the solution is now 
filtered, the filtrate will be found to consist of a con- 
centrated hydrosol. 

Expt. 32. Aluminium hydroxide sol—tThe ionic 
concentration necessary for sol formation is attained by 
evaporation (W. Crum, J. Gann). Diglute a concentrated 
solution of commercial aluminium acetate to twice its 
volume with concentrated acetic acid and heat on a 
water-bath. A granular white precipitate forms in several 
minutes. Decant the supernatant liquid and repeatedly 
wash the precipitate with distilled water ; centrifuge and 
repeat the washing. The precipitate is thereby gradually 
dissolved forming the desired suspension. If this turbid 
suspension is heated, it forms a perfectly clear sol. A very 
concentrated suspension forms only turbid solutions. 
The dilute sols may be concentrated by evaporation 
without becoming turbid. Heat the sols on a water- 
bath for a day to remove the excess of acetic acid. Replace 
the water lost by evaporation. 


14 PRACTICAL. COLLOID, CHEM tata 


For another, more rapid method of preparation, see 
ExXpte 37: 

Expt. 33. Vanadium pentoxide sol (W. Biltz)— 
Triturate in a mortar about 5 g. of ammonium vanadate 
with a few drops of hydrochloric acid. Filter the red 
precipitate and wash it continuously until the filtrate 
assumes a dark red colour. The precipitate is thus 
peptized. Wash it into an Erlenmeyer flask, add about 
200 c.c. of water and stir. Within a few hours a dark red 
solution is obtained which is fairly clear to transmitted 
light. 

Expt. 34. Congo-rubin blue hydrosol formation 
and dissolution of a sol with variation in hydrion con- 
centration. Add HCl to 20 c.c. of a o-I per cent. solution 
of Congo rubin, until precipitation is complete. Allow 
the mixture to stand about 24 hours for complete settling 
of the precipitate. Decant the supernatant lhquid and 
distribute the precipitate between two hard filters. 
The filtrate should be colourless and without any blue 
tinge. If not, wash the precipitate with some dilute 
2N HCl. Finally wash the precipitate on both filters 
with distilled water. Since the precipitate adheres to 
the filter paper, the washings may be hastened by 
decantation, Continue to wash until an intense dark 
blue hydrosol passes through the filter.? 

When both precipitates are in the stage of hydrosol 
formation, add dilute HCl to one filter and water to the 
other. Hydrosol formation ceases very quickly in the 
acidified filtrate which becomes colourless, The non- 
acidified precipitate is in a state of sol formation while the 
other is prevented from sol formation by the excessive 
hydrion concentration. These states are reversible for 
the same precipitate any number of times by changing 


1 Usually the filtrate shows a violet coloration previous to 
actual hydrosol formation. 


ProeeannttON OF COLLOIDAL’*SOLUTIONS 15 


the hydrion concentration. The precipitate which yielded 
a blue filtrate with water will give a colourless filtrate 
with dilute HCl. Conversely, the precipitate which yielded 
a colourless filtrate with HCl will give a blue filtrate with 
water. 

For class demonstration use two porcelain suction 
filters and a water suction pump. 


B. CHEMICAL DISPERSION BY PEPTIZATION 


Expt. 35. Mercury sulphide hydrosol by washing 
and peptizing—Pass H.S into 20 c.c. of saturated 
mercuric chloride solution until the greyish-white pre- 
cipitate first formed, turns black. Filter or decant the 
supernatant liquid and wash repeatedly with distilled 
water. Suspend a portion of the precipitate in water 
and again treat with H.S for about ten minutes. Most 
of the precipitate is thus obtained in colloidal solu- 
tion. 

Expt. 36. Ferric hydroxide sol ; peptization by the 
addition of sol-forming ions. Adda sufficient amount 
of ammonium hydroxide, drop by drop, to a ferric chloride 
solution (5 c.c. of saturated FeCl, solution in 100 c.c. of 
water) until the supernatant liquid is tinged red. Wash 
the precipitate several times by decantation and transfer 
it to an Erlenmeyer flask. Add about 4o c.c. of H.O 
and shake until a thick consistency is obtained. Pour 
5 c.c. of this solution and then about 100 c.c. of distilled 
water into each of five Erlenmeyer flasks. Retain flask 
1 for a control. Adda few drops of 0o-1N HCl to flasks 
2 and 3, and a few drops of the original FeCl, solution to 
flasks 4 and 5. Shake the flasks vigorously. After Io 
minutes the colourless supernatant liquid in each flask 
appears brownish red due to hydrosol formation. Com- 
plete dissolution of the precipitate may be obtained by 


16 PRACTICAL COLLOID CHEMIA Ta 


adding calculated amounts of HClor FeCl;. On standing, 
the hydrosol becomes intensely red. 

Compare this with Experiment I02 on the negative 
ferric hydroxide sol. 

Expt. 37. Aluminium hydroxide sol by peptiza- 
tion with HCl—Precipitate the hydroxide from a dilute 
o-oIN solution of AICI, or Al,(SO,),; with ammonium 
hydroxide. Allow the precipitate to settle or centrifuge 
it. Decant and wash the precipitate several times with 
distilled water. By addition of a few drops of dilute 
HCl, dilution and long heating, the precipitate again 
forms an opalescent suspension which passes through 
a baryta filter. The sol is more coarsely disperse than 
those in Expts. 23 and 32, which gave practically clear 
hydrosols. 

Expt. 38. Prussian blue by peptization with 
oxalic acid—Add about 5 c.c. of saturated aqueous 
K,Fe(CN), to Io c.c. of 0-05 m. FeCl;, filter and wash 
the precipitate. Pour several portions of 0-1N oxalic acid 
over the precipitate. The resulting filtrate constitutes 
the desired blue coloured sol. 

Expt. 39. Stannic acid hydrosol by peptization 
with ammonium hydroxide—Prepare a dilute solution 
of stannous chloride by dissolving I g. of SnCl,.2H.O in 
300 c.c. of H,O. Pour this solution into a filter flask 
fitted with a stopper and a long glass tube. Draw air 
through the solution for about two days by means of a 
suction pump. The turbid solution becomes clear and 
then turbid again. Continue to draw air through the 
solution until a precipitate forms. Decant and wash 
(R. Zsigmondy). This procedure requires several days 
if dilute solutions are used. To hasten the process, 
warm the solution as soon as it has become turbid by 
aeration in order to obtain the coarse flocculent pre- 
cipitate. Decant and filter the supernatant liquid. 


PitteeanatiQGN OF COLLOIDAL SOLUTIONS 17 


Wash the precipitate with hot water until the washings 
give no test for chlorides. Add a few drops of dilute 
ammonium hydroxide, warm until the precipitate dissolves 
to form a clear solution of stannic acid hydrosol. Heat 
to drive off excess ammonia. 

The sol is obtained more quickly if anhydrous stannic 
chloride is used. Add, drop by drop, a solution of stannic 
chloride to a relatively large volume of water. Wash the 
precipitate and add ammonium hydroxide as above to 
obtain the desired sol. The sol may be prepared very 
conveniently by using the hydrate, SnCl,.5H.O. Add, 
drop by drop, a 5 per cent. solution of this salt to boiling 
water. Compare this with the preparation of colloidal 
fer jewwydrexide in Expt. 20. The thick flocculent 
precipitate forms immediately and gives a hydrosol when 
treated as above. This procedure takes less than an 
hour if the washing is hastened by the use of a porcelain 
funnel and suction pump. 

The sol may be coalesced by the addition of neutral 
salts, caustic alkalies or dilute acids. A typical property 
of these sols is the ease with which they foam. 

To prepare stannic acid hydrosols of different degrees 
of dispersion, see M. Mecklenburg, Zeztschr. f. anorg. 
Chem., 74, 207 (1912). 


ELECTROLYTIC METHODS OF DISPERSION 


Expt. 40. Preparation of colloidal’ metals and 
metallic oxides by electrical dispersion—This 
method consists in the disintegration of wire electrodes 
by producing an electric arc between the ends. Insert 
two silver wires 0:-5-I mm. in diameter in narrow glass 
or rubber tubes for insulation Connect the wires with 
a 110 volt direct current and introduce a variable resist- 
ance in the circuit. The current flow should be 4 to 12 

2 


18 PRACTICAL COLLOID’ CHEMIST 


amperes. Place the two electrodes about 2 or 3 mm. 
apart in a porcelain dish or glass beaker containing water 
and ice. One or both electrodes may be held to a small 
support. Regulate the current so that a green coloured 
arc forms between the ends of the electrodes when they 
are brought near one another. Dark brown or olive- 
green clouds of dispersed metallic and oxidized silver 
form in the water. Colloid formation is favoured by 
adding a few drops of 2 per cent. Na.CO, solution to the 
water. Silver and platinum disintegrate very readily, 
gold with difficulty. 

Expt. 41. Preparation of colloidal lead by 
electrolysis (G. Bredig and F. Haber)—A lead plate, 
fitted with a binding post, is used for the anode. A 
lead wire, I-2 mm. diameter, is suspended on a support a 
short distance from the plate, for the cathode. Immerse 
both electrodes in a 1 per cent. solution of NaOH contained 
ina beaker. Passa direct current of 220 volts through the 
system. The lead wire disintegrates to form black 
clouds of lead particles, partly colloidal. Oxidation and 
ficcculation of the dispersed lead particles removes them 
from the colloid state. 

Expt. 42. Lead pyrosol (R. Lorenz)—Fuse some 
well-dried lead chloride. Throw some shavings of pure 
sheet lead into the melt and observe : first, the lead par- 
ticles become surrounded with a crust of solid lead 
chloride and fall to the bottom of the tube ; second, the 
salt melts and the lead particles coalesce into a molten 
drop similar to that of mercury. At the same time, 
dark brown clouds of highly dispersed lead begin to 
form. 

It is not known whether the sol formation is due to 
a condensation of a molecular disperse vapour or a 
colloidal solution of lead in the melt. 


Pie LON OF COLLOIDAL SOLUTIONS 109 


EXPERIMENTS ON THE THEORY OF FORMATION 
OEeeOLEOIDAL SOLUTIONS 


Von Weimarn’s rule states that the size of precipitated 
particles is greatest when medium concentrations of the 
two reacting solutions are used. With very low or very 
high concentrations the precipitated particles are smaller. 

Expt. 43. Prepare saturated solutions of FeCl, and 
K,Fe(CN),. Mix the two solutions in the following 
concentrations : 

(a) Add 1 c.c. of K,Fe(CN), and 2-4 drops of FeCl, 
to 200 c.c. of H,O. A very clear permanent sol forms. 

(eerddet cc. of K,be(CN), and 1 c.c. of FeCl, 
to 200 c.c. of H,O. A voluminous precipitate settles 
in about half an hour from the deeply coloured solution 
and the bright blue supernatant liquid becomes almost 
colourless on standing. 

(c) Pour ro c.c. of the saturated ferro-cyanide solution 
in a small beaker ; stir and add 2 c.c. of saturated FeCl, 
solution drop by drop. A paste forms which is so viscous 
and adherent to the beaker that none is lost on inverting 
it. Take a small amount of this precipitate on a glass 
rod and put it in about 200 c.c of water. A clear per- 
manent hydrosol forms. 


Se bitityYeAND MOLECULAR SOLUBILITY OF A 
BOLLO@LDAL SYSTEM 


The greater the molar solubility of a precipitate, the 
less stable it is when in the colloidal state. Since the 
larger particles grow at the expense of the smaller, a 
precipitate with a larger molar solubility will change from 
a finer into a coarser form. In analytical chemistry, a 
precipitate, which is difficult to filter, is brought to a 
coarser state by increasing the solubility and thereby 


20 PRACTICAL COLLOID CHES. 


hastening the growth of the particles, by heating for a 
certain length of time. On the other hand, under similar 
conditions, the precipitate may occur in a colloidal form 
when the molar solubility is comparatively small. 
Expt. 44. Solubility of silver halides at 20° C. 


AgCl: 0-0016 g. per litre 
AgBr: 0:000084 g. per litre 
Agl:  0-:0000028 g. per litre 


—Dilute 5 c.c. each of tenth-normal solutions of KBr 
and KI with 100 c.c. of H,O. Add 3 c.c. of o-1IN AgNO; 
to each of the three solutions with continuous shaking. 
The precipitates form sols consisting of relatively coarse 
particles, which coagulate after 1-2 days. ‘The less the 
solubility of the precipitate, the more rapid the settling. 


COLLOID FORMATION IN THE PRESENCE OF PRO- 
TECTIVE COLLOMs 


Highly disperse and reversible colloids are stabilized 
by preparing the colloid in the presence of especially 
hydrated emulsoids such as gelatin, egg white, tannin, 
dextrin, etc. A reversible colloid is one which becomes 
solid upon evaporation and is spontaneously soluble 
when water is added again. A well-known technical 
example is the preparation of collargol. The following 
experiments show the influence of a protective colloid 
upon the formation of a highly disperse colloid. 

Expt. 45. Add 2 drops of o-1IN AgNO; to 20 c.c. of 
H.O acidified with 4 drops of HNO, and divide the solu- 
tion into two equal portions. Add to the first portion 
I c.c. of 2.05 per cent. solution of gelatin and to the second, 
I c.c. of water and shake. Add 3 drops of 0-IN HCI to 
both solutions and shake again. The second solution 


fee ONT OF COLLOIDAL SOLUTIONS ar 


becomes white and turbid while the first remains clear. 
A faint opalescence appears in the first solution after 
several minutes. This increases after a few hours, but 
no precipitate forms as in the second. 

Expt. 46. Add 5 c.c. of a saturated PbCl, solution 
to I0u c.c. of water. Divide the mixture between two 
Erlenmeyer flasks. Add 3 c.c. of a 1 per cent. freshly 
prepared gelatin solution to one flask and pass H,S into 
both mixtures for a few minutes. The solution without 
the gelatin gives a flocculent precipitate, while the other 
forms a permanent dark brown lead sulphide hydrosol. 
A thin layer of this solution appears perfectly clear. 

Expt. 47. Preparation of photochloride sols— 
Prepare two sols of AgCl according to Expt. 45 and expose 
both to sunlight or a strong arc light. Sol A gives a 
beautiful, clear, red-brown, highly disperse photochloride 
of AgCl and colloidal silver. Sol B gives a turbid, violet- 
grey coarsely disperse chloride. 


1 Of course the gelatin used must be washed free from chlorides. 


II 


DIFFUSION, DIALYSIS, ULTRA- 
FILTRATION 


DIFFUSION 
(_ J ise at solutions show little diffusion. Quanti- 


tative determinations of the diffusion coefficients 

are difficult. Dilute jellies of gelatin, agar-agar, 
or silicic acid influence the speed of diffusion but slightly 
in semi-quantitative determinations of the rate of diffusion 
in various disperse systems. 

Expt. 48. Prepare a hot 3 per cent. solution of gelatin 
and pour it half-full into a series of test-tubes. Allow it 
to solidify at room temperature.t Pour various solutions 
over the jelly. The diffusion of coloured solutions can 
easily be followed as they penetrate the jelly. Colourless 
solutions, however, require a reagent previously dissolved 
in the jelly, as NaCl with diffusing AgNO,.2 An alter- 
native is to heat for a short time and draw out the jelly - 
into long threads. Wash carefully with ice water, cut 
into long strips and allow the reagent to act upon these 
strips. 

A thin strip of graduated paper pasted over the length 
of the test-tube will aid in the estimation of the diffusion 


1 Jn the first case a loosening of the gelatin cylinder arom? the 
sides of the tube may occasionally be observed. 
2See Expt. 128 on the irregularities which occasionally occur 
in such diffusion reactions. 
22 


Porn DAL YSIS, ULTRAFILTRATION 23 


coefficient, expressed in millimetres per twenty-four 
hours. | 
Determine the diffusion coefficients of the following :— 























| 
RATE OF DIFFUSION. | 
= : i ; z ae | 
Pyactically Zero. | Fairly Rapid. | 
Black India ink Methyl violet | 
Gold or silver sol | Acid fuchsine | 
Ferric hydroxide sol Brilliant green 
Mercury sulphide sol Methylene blue 
Prussian blue Saturated copper sulphate 
Alkali blue | 
Congo red 
Slow. 
Congo rubin 








The determination of the rate of diffusion detects 
“ transition-systems,” i.e. substances in a state between 
truly colloidal and truly molecular. Thus, Congo rubin 
shows considerable diffusion after 3 or 4 days, while 
Congo red only 1 to 2 weeks. 

Expt. 49. Diffusion analysis of polydisperse 
systems—aAdd sufficient dilute alkaline aqueous eosin 
to a solution of “ night blue ” until the mixture appears 
dark violet. The red eosin penetrates the jelly in twenty- 
four hours, while the “ night blue ”’ does not diffuse. 

Other colour mixtures suitable for such experiments 
aver: 

(a) Alkali blue 4- picric acid. The yellow dye 
diffuses out of the green mixture. 

(6) Alkali blue + acid fuchsine. The red dye diffuses 
out of the blue-violet mixture. ne: 


24 PRACTICAE COLLOID CHEMT aes 


Expt. 50. Add silver nitrate to chloride free gelatin 
before solidification and divide in two parts. Pour 
commercial ferric hydroxide sol over one portion of the 
formed jelly and the sol well purified by dialysis over the 
second portion. A silver chloride precipitate occurs in 
the first but not in the second. 

Expt. 51. Add a few drops of phenalphthalein to 
some aqueous NaOH. Pour this into a warm solution of 
gelatin in order that the jelly may appear red. Pour 
over the jelly some slightly acidified “ alkaline blue.”’ 
The rapidly diffusing hydrion decolorizes the jelly and 
produces three patriotic layers: violet, colourless and 
red. 


DEADY Sis 


A comprehensive description of a large number of 
dialysers is found in Abderhalden’s Handb. d. biochem. 
Arbeitsmethoden, III, 1, pp. 165-80, 1910, by E. Zunz ; and 
in Weyls-Hauben Methods of Organic Chemistry, 2nd 
edition, 1921, I, 428, by H. Rheinboldt. 

Dialysers serve for : 

1. Direct preparation of colloids. 

2. Purification of large amounts of colloid material. 

3. Colloid analysis, as in the determination of the 
dialysing properties of sols. 

Graham’s bell dialyser consists of parchment paper 
stretched over a glass cylinder.1 R. Zsigmondy’s star 
dialyser consists of durable parchment cups.? However, 


1 To seal the dialysing membrane to glass, use sealing wax, 
Chatterton rubber compound, Canada balsam, Jordis adhesive 
cement consisting of equal parts of collodion and concentrated 
alcohol or a 50 per cent. shellac solution. 

2 The parchment cups should not hang free, but be placed either 
at the bottom of the vessel or supported at the side of the dialysing 
vessel to prevent tearing. 


Pe ram eOIAL YSIS; ULTRAFILTRATION 25 


pig bladders with the fat removed by ether are also used 
for dialysis. 

Expt. 52. Simple dialysers for preparation of 
colloids—Cut out a large round sheet of parchment 
paper, and fold it over an Erlenmeyer or round-bottomed 
flask so as to form a sac. Pour the colloid into a sac, 
tie it with a cord and suspend it in distilled water. Change 
iieewvetereireoquentiy. Place commercial 5 per cent. 
ferric hydroxide in the dialyser and after 1-2 hours 
determine the amount of chloride present in the water by 
means of silver nitrate. This reaction disappears after 
dialysing a sufficient length of time and flocculation of 
the sol usually occurs. Fig. 1 illustrates a better form 
of dialyser. Punch holes along the edge of a circular 
sheet of parchment paper. Pass two threads through 





26 PRACTICAL COLLOID CHEMIsSEie® 


these holes similar to the drawstrings of a pouch. Moisten 
the paper, fold in the shape of a sac and close by pulling 
the threads. 

Expt. 53. Make a large folded filter from parchment 
paper and place it in a dialysing funnel. Fasten a glass 
tip to the lower end of the funnel by means of a rubber 
tube fitted with a pinchcock. Pour the colloid about 
two-thirds full into the filter and cover with a glass plate 
or watch glass. Close the pinchcock and fill the space 
between the filter and the walls of the funnel with distilled 
water. Allow water to flow continuously into the funnel 
and regulate the pinchcock so that the water flows out 
at the same rate. By this method, comparatively large 
quantities of colloid may 
be dialysed rapidly. The 
pinchcock is not necessary 
if a funnel is used whose 
tube has a double bend, 
as in’ Fig. TAS eel ere 
should be a fairly large 
perforation at the site in- 
dicated. The water in the 
funnel always has _ the 
same level as that outside, since the perforation pre- 
vents siphoning. Place a concentrated solution of com- 
mercial ‘‘ night blue ”’ in the dialyser and detect, by means 
of BaCl,, the gradual disappearance of the Na,SO, which 
is removed by dialysis. 

These two simple dialysers may be heated at higher 
temperatures to obtain more rapid dialysis. 

Small pieces of parchment skin are best for colloid 
analysis. Those prepared from the cecum of sheep are 
light and thin and may be shaped in a funnel. 

Expt. 54. Simple dialysers for colloid analysis— 
Take large extraction thimbles and moisten them with 





ATA 


PiU SON DIALYSIS, ULTRAFILTRATION 27 


warm distilled water. Drain off excess of water and while 
still warm fill one with a 4 per cent. collodion solution. 
Empty this collodion immediately into a second thimble 
and continue emptying into athird thimble, etc. A thin 
collodion layer on the wall of the cup is thus obtained. 
Keep all thimbles inverted so that no drops of collodion 
adhere to the bottom. Dry for about five minutes. 
Prepare in the same way, a second very thin layer, taking 
special care to remove excess collodion. After 5-10 
minutes, immerse the thimble into cold water for about 
thirty minutes. These dialysers should be kept under 
water when not in use. 

Prepare a 2 per cent. collodion solution by diluting the 
commercial collodion with an equal volume of 7 parts 
ether and 1 part absolute alcohol. Collodion of such 
strength forms very thin and rapidly acting dialysers. 

Test the dialyser with dilute 0-05 per cent. “ night 
blue’ solution. 

These dialysers are very durable and may be washed 
in flowing water by means of a wad of cotton. Heat 
does not destroy them. 

Large extraction thimbles have a capacity of about 
200-300 c.c. and dialysers may be made from these. 

Expt. 55. Demonstration of the dialysis rates of 
dyes—Place three analytical dialysers in small beakers 
containing distilled water. Fill them three-fourths full 
of “ night blue,’ Congo rubin and picric acid respectively. 
After twenty-four hours the “ night blue’ does not pass 
through at all; the Congo rubin but slightly and the 
picric acid to a marked degree. 

Alternative :—Dialyse the mixture of I per cent. “ night 
blue” and alkaline eosin prepared in Expt. 49. The red 
dye appears in the outer wash water while the “ night 
blue’ remains in the dialyser, 


28 PRACTICAL COELOID* CHESS ia 
FILTRATION 


A typical colloid, apart from adsorption, will pass un- 
changed through a filter paper. Hence, filter paper, clay 
filters, etc., are used in analytical work for the separation 
of typical colloids from coarse dispersions. The following 
table gives a list of filters with the size of pores in each. 

The numbers indicate the diameter of the largest pores. 
The filters Nos. 602 (hard) and 602 (extra hard) are the 
so-called baryta filters, i.e. those which partially hold 
back freshly precipitated BaSO, and CaC,O,. 


SIZES OF PORES OF FILTER PAPER. 
(H. Bechhold and R. Lucas.) 


1450 about 4:8 

598 Ness): 

thick filter paper Pr pec 

597 =e 

602, hard eee ay 

566 » «17M 

602, extra hard Teese 

Chamberland-Kerze 5, O72-O0'4u 

Reichel-Kerze » O'16-0:18u 


Expt. 56. Filtration of a heterogeneous disperse 
colloid—Add very dilute HCl or HC,H,0, to a o-r per 
cent. solution of Congo rubin until the solution just 
assumes a violet tinge. This solution passes through 
ordinary filter paper. Only the red or faint violet 
solutions pass through filter paper of 1-5 in diameter. 
The filtrates turn violet on addition of concentrated acid. 


ULTRAFILTRATION 


Expt.57. Asimple ultrafilter for colloid analysis. 
Fit carefully a smooth piece of filter paper into a clean 


PIP EUStON, DIALYSIS, ULTRAFILTRATION 29 


funnel, moistened well with hot water. Allow to drain. 
Pour 20 to 30 c.c. of warm 4 per cent. collodion over the 
damp filter. Rotate the funnel as rapidly as possible to 
obtain a primary coat of collodion upon the paper. The 
collodion should coat the surface of the filter paper but 
once, for the thicker the membrane the slower the filtration. 
Drain the excess collodion by inverting the filter so that 
no drops adhere to the tip. Allow to dry in the air for 
about five minutes. Remove the hardened filter paper 
from the funnel. Place the filter into distilled water 
for thirty minutes before using. 

A more convenient and efficient filter may be prepared 
by using extraction thimbles as a support for the collodion 
membrane. ‘The thimbles are of various sizes and filters 
of 200 to 300 c.c. capacity may be prepared. 

Wash the filter with distilled water before using, in 
order to remove the traces of coagulated collodion. 
Test the filter with dilute “night blue” solution of 
mastic hydrosol. ‘The filters are very durable and may 
be used repeatedly. Wash under a slow stream of water 
with a wad of cotton. 

Such ultrafilters are spontaneous, that is, they filter 
under the pressure of their contents. It is difficult to 
prepare a filter of about 100 c.c. capacity which filters 
more rapidly that 1 to 2 c.c. per minute for colloidal 
solutions of the same viscosity as water. Place the 
ultrafilter in a funnel with a 60° cone and connect it with 
a section pump to hasten filtration. Too great a suction 
tears the point of the filter. 

Use a 4 per cent. solution of collodion diluted with a 
mixture of 7 parts ether and 1 part alcohol, to prepare 
ultrafilters with a greater permeability. These mem- 
branes are thinner. A 2 per cent. collodion solution is 
best suited for most purposes. This provides a more 
permeable filter which is, however, impermeable to a 


30 PRACTICAL COLLOID CHEMIST: 


“night blue’ solution. The collodion containing 
greater concentration of alcohol may be heated to a higher 
temperature. Test the permeability of the filter with 
several standardized freshly prepared colloidal solutions 
of different degrees of dispersion such as ‘‘ night blue,”’ 
Congo red, and collargol. A filter prepared from 4 per 
cent. collodion gives a clear colourless filtrate with these 
three dyes. A filter prepared from a 3 per cent. collodion 
solution retains “ night blue’’ and often Congo red. A 
filter prepared from 2 per cent. collodion retains none 
but “night blue.” These differences in permeability 
are due to the alcohol concentration of the collodion. 
Expt.58. Preparation of a suction ultrafilter. A 
Buchner funnel is suitable for the preparation of an 
ultrafilter with a large surface. This ultrafilter may be 
used with suction. Select a funnel with a smooth bottom. 
Prepare a 2 per cent. ether solution of crude rubber. 
Allow the solution to stand in a warm dark place, for light 
decomposes it. Pour about 2°¢.c. Of theveieamamraues 
solution along the edge of the inclined funnel and rotate 
slowly. The solution yields a very thin rubber ring 
after evaporation of the ether. The rubber band serves 
the purpose of a binding rim between the porcelain and 
the collodion membrane, thereby ensuring a water-tight 
system. Collodion does not adhere to porcelain or glass. 
Place the funnel in a horizontal position and fit with a 
dry sheet of a medium-pored filter paper. Moisten the 
sheet with distilled water and carefully press the filter 
paper against the rim, keeping it free from wrinkles. 
Allow the paper to dry so as to decrease the tendency 
to warp. To remove the last trace of water, incline the 
funnel and absorb the collected water with filter paper. 
Take precautions not to tear the rubber band during the 
washing. Keep the funnel in a horizontal position and 
pour warm collodion over the paper in the manner 


MiP stONe DIALYSIS, ULTRAFILTRATION 931 


described above. Remove the last drops of excess 
collodion by inverting the funnel. A thicker collodion 
layer remaining in the concave edge produces leaks. 
After 5 to 10 minutes, pour the second collodion layer, 
with precautions to remove excess collodion. Dry for 
Io minutes and add no more than 2 c.c. of distilled water 
to coagulate the membrane. Too much water will exert 
a greater pressure upon the soft “‘ spongy ’’ membrane 
and may destroy the efficiency of the filter. The first 
drops of the filtered water contain collodion. Test with 
“night blue” or mastic hydrosol. The filter may be 
washed with water and will last for months. 

Rapid filtration is obtained if suction 1s applied to the 
ultrafilter on the Buchner funnel. The author could 
filter 200 c.c. of perfectly clear filtrate from a “ night 
blue” solution in i minute. The ultrafilter lasts longer 
and its efficiency is greater when less suction is ap- 
plied. 

Expt. 59. Ultrafiltration of disperse colloids— 
Prepare several ultrafilters and set up a funnel with a 
paper filter alongside of each ultrafilter. Fill an ultra- 
filter and a paper filter with the same solutions used in 
Expts. 48 and 55. “‘ Night blue,” dilute India ink, 
dialysed ferric hydroxide, etc., pass through the filter 
paper unchanged. The ultrafilter retains completely the 
colloidal particles. Congo red and collargol are com- 
pletely or partially held back, depending upon the per- 
meability of the filter. A dilute Congo-rubin solution 
produces a clear filtrate at first, then the dye begins to 
appear in the filtrate. Molecular disperse systems, such 
as acid fuchsine, picric acid, methyl violet, etc., ulti- 
mately pass through an ultrafilter as easily as through 
filter paper. 

Expt. 60. Separation of colloids and molecular 
_ disperse phases by ultrafiltration—Add some NaCl 


32 PRACTICAL COLLOID CHEM iia 


or Na,SO, to a “night blue’’ solution, ultrafilter and 
test the filtrate for chlorides and sulphates. 

Ultrafilter commercial Fe(OH); and dialysed Fe(OH) s. 
Commercial Fe(OH); gives a yellow filtrate which is 
positive for ferric and chloride ions. Dialysed Fe(OH); 
gives a clear colourless filtrate which is but slghtly 
positive for both ions. 

Expt. 61. Separation of dye mixtures by ultra- 
filtration—Filter the dye mixtures used in Expt. 43 
(“night blue”? + alkaline eosin; “alkali blue” + 
picric acid or acid fuchsine) through a paper filter and 
through an ultrafilter respectively. The freshly prepared 
mixtures pass unchanged through the filter paper. The 
dark violet mixture gives a bright red filtrate with the 
ultrafilter; the blue or dark green mixture a bright 
yellow filtrate and the blue-viclet mixture, a red filtrate. 

Expt. 62. Ultrafiltration analysis of a three- 
phase heterogeneous disperse mixture—Add sufficient 
“alkali blue” to 200 c.c. of a solution of colloidal 
graphite in order to give the supernatant liquid a bright 
blue colour. 

The colloidal graphite coagulates to form a coarsely 
disperse system. Now add saturated aqueous picric acid 
until the solution becomes green. Shake the mixture 
until the graphite gives the solution an opaque black 
colour. Filter three portions of this solution into 
Erlenmeyer flasks. The first funnel should contain a 
filtering cloth, the second a filter paper, and the third 
an ultrafilter. The filtrate in the first flask consists of 
the unchanged black mixture ; the second filtrate, of a 
green dye mixture freed from the graphite, and the third 
filtrate, of the yellow picric acid separated from the 
“alkah blue.” 

Expt. 63. Use of ultrafiltration in the deter- 
mination of a small variation in degree of disper- 


Moree slON DIALYSIS, ULTRAFILTRATION 33 


sion. Observe that a neutral solution of Congo rubin 
gradually passes through an ultrafilter. Add a trace of 
acid to a 0-OI-0'I per cent. solution of Congo rubin to 
produce a blue tinge. The solution appears perfectly 
clear to the naked eye. Ultrafilter this blue solution. 
The ultrafiltrate is completely colourless or has a rose 
to violet tinge, depending upon the amount of acid added 
and the age of the blue solution. 


III 
SURFACE TENSION AND VISCOSITY | 
SURFACE TENSION 


"T's simplest apparatus for the determination of 

surface tension of colloidal solutions is a stalag- | 
mometer. It consists of a calibrated pipette 
with a capillary tube sealed to one end. The capillary 
has a fine enough bore, so that water flows out drop by 
drop when the tube is full. The free end of the capillary 
has a flat circular surface. A fluid of no surface tension 
would flow from this tube with a steady stream. Surface 
tension renders this impossible and causes the flowing 
liquid to accumulate into a drop which breaks when 
the weight exceeds the retaining force exerted by its 
surface tension. This force of retention is equal to the 
surface tension times the circumference of the drop. The 
greater the surface tension the greater will be the size of 
the drop. The number of drops of liquid obtained from 
a known volume of the same solution, obviously, is a 
measure of the surface tension of that liquid. With large 
drops the number is correspondingly less for a given 
volume of liquid and the surface tension is proportion- 
ately greater. Using the drop number of water for com- 
parison, an increase in that number denotes a decrease in 
the surface tension, while a decrease in the drop number 
denotes a corresponding increase. This method gives 
relative values for surface tension, adequate for most 


studies of colloidal solutions. 
34 


SURFACE TENSION AND VISCOSITY 35 


Surface tension decreases but slightly with increase of 
temperature, therefore temperature control is _ not 
necessary for most experiments. On the other hand, 
the surface tension varies with the rate of dropping, 
because the drop number tends to be greater 
for a slower rather than a faster rate of 
flow. Equal rates of the dropping are 
necessary for accurate measurements. 

Take a 3 or 5 c.c. graduated pipette and 
attach a piece of capillary tube to one end by 
means of a rubber connection. The capillary 
must be of sufficient length and of suitable 
bore so that the liquid flows by drops when 
soem pipette is full. Lhe free end of the 
capillary must be ground flat. Suspend the 
pipette at such a height that a dish or an 
Erlenmeyer flask may conveniently catch the 
falling drops. A greater number of drops 
per unit volume, 124 instead of 120, is ob- 
tained when the pipette isinclined. Fill the 
pipette above the upper mark and _ begin 
counting the drops released when the 
meniscus just passes it. The accuracy of 
the results increases with the increase of 
absolute drop number, hence the volume 
used should yield more than roo drops. 

Expt. 64. Surface tension of soap 
solutions—Prepare a 0-1 per cent. solution 
of any soap. A soap solution gives a drop 
number of 90 or more, while pure water 
gives about 60 per unit volume. Measure 
the drop number of intermediate concentrations to deter- 
mine the minimum concentration of soap necessary to 
produce a considerable decrease in surface tension. 

Expt. 65. Stalagmometric studies of colloid 


Fic. 2° 


36 PRACTICAL® COLLOID CHERIi Sas 


chemical reactions (J. Traube).—Measure the drop 
number of a o-2 per cent. “‘ night blue”’ solution. It is 
higher than that of pure water. Add the following 


amounts of KI to 10 c.c. of “ night blue ”’ solution : 
Examples 
of Drop 
Substance added. Number 
Values. 
Water Z : — 124 
0-2 pos cents a bine : — 144 
7O0.C.G: snightp phe: ta. . + 5 drops o-oo1IN KI 142 
~ oe ; . + 1 drop o-o1N - 140 
- be , . + 2 diopsto-OnN wae 130 
- i : . +5 4, 20-Oriaa 120 
* F : . + 1drop oN be 119 
4 1 ; . 11 oe 5 127 
- re : . + 2 drops o-iia 126 
3 “ é + 6 5 OLN - 125 
Moceute tiene 
Water ‘ : : : — 124 


The following numbers are obtained by using KBr 
instead of KI : 


Drop 


Substance added. Number. 
Water : a 124 
O-2.per cents. niehe ues : — 144 
10.6:¢.- “night bites ae . + 1 drop hier 137 
PS % : .. 4a Be 124 
‘ Me : - = bbe . . 119g 
a : . + 2 drops an 117 
*: . ile pote if 116 
a 5 eee A 131 
ss 5 . i Die - 128 
¥ ‘ . ape! Fe 127 
rs a . ~ po Cae ‘ 126 
» 3 ‘ + 30 ,, ry 125 
Flocculaaeas 


The effect of alkali halogens upon the surface tension 


SURPAGE TENSION AND VISCOSITY 37 


of “ night blue ” is shown in Fig. 3. Flocculation of the 
dye occurs just before the drop number for water is 
reached. 


Drop Number >> —— na KJ 


Substance Added 





100 200 200 400 500 600 = 00 


Fic. 3. 


VisCOsliy 


Viscosity denotes the resistance which a fluid exerts 
against displacement of its own molecules. Glycerin has 
a high viscosity and ether a very low one. An approxi- 
mate measure of this value is the time a given volume of 
liquid requires to flow through a certain capillary. 
Relative viscosity suffices in colloid chemistry. It is 
proportional to the product of the time of flow, the 
specific gravity of the substance and the so-called ap- 
paratus constant, K. This is determined by standard- 
izing the viscosimeter in terms of distilled water. Gener- 
ally, the specific gravity may be neglected. If the 
viscosity of water, observed in the apparatus, be as- 
sumed unity, the relative viscosity of the colloid studied 
is simply the ratio of the time of flow of the colloid to 
that of water through the viscosimeter. The viscosities 
of many homogeneous liquids are independent of the 


38 PRACTICAL COLLOID CHEMISE: 


pressures which produce the capillary flow. However, 
the viscosity is affected appreciably at higher pressures, 
of course in the direction of a faster rate of flow. The 
work of E. Hatschek, W. Hess, and E. Rothlin shows 
that the viscosities of hydrophile colloids deviate from 
Poiseuille’s law with slight variations. Hence, even 
relative viscosity measurements must be made at known 
or constant pressures. 

The Ostwald viscosimeter is a very convenient form 
of apparatus. It consists of a U-tube, one arm 
of which has a capillary and bulb. The two 
ends of the bulb are marked so that it may 

J containa definite volume (Fig. 4). In using the 
viscosimeter, always place the same volume of 
liquid in the tube, draw it into the capillary 
side arm above the upper mark, and measure 
the time of flow of the liquid between the two 
marks. Use a viscosimeter with a capillary of 
greater bore for viscous liquids, such as hydro- 
phile emulsoids. Viscosimeters whose capil- 
laries permit the volume of water between the 
two marks to flow out within 20 seconds are 
most suitable. 

The viscosity of a liquid varies considerably 
with the temperature, hence the viscosimeter 
should always be kept in a water-bath with two glass 
sides so that the flow of the lquid may be observed. 
The viscosimeter should be cleaned by drawing cleaning 
mixture and water through the capillary. The presence of 
gas bubbles in the capillary prevents adequate cleansing 
of the viscosimeter wall, and therefore inaccurate meas- 
urements result. This viscosimeter may be more con- 
veniently used by providing the side arm with a rubber 
stopper containing a bent glass tube. Attaching a piece 
of rubber tubing to the glass tube, the liquid within 


BiG. A; 


SUR AGH. CE NSION AND VISCOSITY 39 


the viscosimeter may be blown into the capillary and 
back with each measurement. 


Wee oti yY sex PERIMENTS. WITH GELATIN 
SOLUTIONS 


Gelatin is a hydrophile emulsoid which has been studied 
more extensively than other albuminous substances. 
Prepare a I per cent. solution of gelatin in the following 
manner: Weigh 2 g. of either sheet gelatin, gelatin 
cuttings used for photographic purposes or gelatin 
powder and place it in cold distilled water. Change the 
water often in order to obtain a “ pure”’ solution. Weigh 
a beaker, add 150 c.c. of water and heat to boiling. 
Remove the flame and add the swollen gelatin free from 
wash water. Stir continuously with a glass rod until the 
gelatin dissolves. The decomposing effect of heat as 
well as the stickiness of the gelatin are thus avoided. 
After the gelatin dissolves, cool and weigh the beaker and 
contents. Add enough water to obtain 200 g. of a I per 
cent. solution. 

Pass water at room temperature through the viscosi- 
meter at least 30 to 40 seconds before measuring the 
viscosity of the gelatin. 

Expt. 66. Influence of the age of gelatin solutions 
upon viscosity—Measure the viscosity of the cooled 
gelatin solution directly after preparation. Measure 
it again in half an hour. The solution may be left in the 
viscosimeter, but must be drawn two or three times 
through the capillary before making a measurement. 
The viscosity of the solution increases considerably after 
standing an hour. A I per cent. solution is usually too 
viscous to flow through the viscosimeter. 

Expt. 67. Influence of preliminary mechanical 
treatment on the viscosity of gelatin solutions— 


40 PRACTICAL COLLOID CHEMTS irae 


Allow a o-5 per cent. gelatin solution to stand in the 
viscosimeter for 24 hours. Measure the viscosity after 
drawing the solution very slowly into the capillary. Then 
agitate the solution by rapidly drawing it through the 
capillary several times or by bubbling air through it. 
Measure the viscosity once more and observe a considerable 
shorter time of flow. 

The structure of a very dilute gelatin solution is the 
cause of these phenomena. Therefore, it is necessary to 
run the solution through the viscosimeter two or three 
times before determining the viscosity of the colloid. 

Expt. 68. Influence of preliminary thermal treat- 
ment on the viscosity of gelatin solutions—Measure 
the viscosity of a 0-5-1 per cent. gelatin solution. Place 
about 200 c.c. of this solution in an Erlenmeyer flask and 
heat on a steam-bath. Provide the flask with a reflux 
condenser to prevent loss of water by evaporation, or 
mark the original water level, and after heating add the 
required amount of water. Remove every half-hour 
about 20 c.c. of solution and determine after cooling the 
viscosity of this sample.! Do not forget to replace the 
water lost by evaporation. To obtain more accurate 
results, heat a larger volume of gelatin solution and take 
larger test portions. Use these portions to determine 
the increase of viscosity with time. Plotting the viscosities 
obtained as ordinates against the age of the solutions 
as abscisse, a series of curves are obtained in which 
the gelatin solutions which have been heated the longest, 
show the lowest slope. For longer periods of heating 
the gelatin solutions, the slopes of the age curves 
approach zero. 


1 The concentration of the solution can be maintained fairly 
constant by weighing the filled flask before and after heating 
as well as before and after removing samples and subsequent 
refilling. 


Seer TENSION AND VISCOSIFY AI 


Expt. 69. Influence of concentration upon the 
viscosity of gelatin solutions—Prepare the following 
solutions by mixing a warm I per cent. gelatin solution 
with warm water, or place the cold mixture on a steam- 
bath for 5 minutes and then cool : 





iinet ey. , : : Te ees eA BS 
So wotereiatin —. : fee a Oe 
cer water |. : eee Sel Ose 5 ris 


Allow the mixtures to stand a few hours before determin- 
ing the viscosity and then determine them all in sequence. 
Plot the curves to show the increase of viscosity with 
concentration and determine the gradual increase in 
slope. 

Expt. 70. Influence of temperature on the viscos - 
ity of gelatin solutions—Determine the rate of flow 
of a 5 per cent. gelatin solution at the temperature of ice 
water. The filled viscosimeter should be allowed to come 
to this temperature by letting it stand at least half an 
hour in the ice water. Then heat the bath to a tem- 
perature of 20°C. and allow the filled viscosimeter to 
remain one half-hour before repeating the determination. 
Make a third measurement at 4o°C. Plot the corre- 
sponding curves and observe the rapid fall of viscosity 
with rising temperature. 

Expt. 71. Influence of additions of electrolytes 
on the viscosity of gelatin solutions—Prepare the 
following mixtures : 


Peon per cent, gelatin -+- 20°c.c. H,O 


oa * * Pees 20rc,. ce IN Na oO) or KaO) for 
; MgsO,. 
2, ay = pe et20 GC. OwINeK IT or K Br, 
4a. a * Peale C:C.t1 Oi 216. C.Or1N. HCl 
| = about 0-005 N HCl. 
4b. oo wo se 10 C.Ce1.0 = 47¢.c7 N’ HCP 


about o-IN HCl. 


42 PRACTICAL COLLOID CHEMIST 


(5%. 20 C.c. I per cent. gelatin4- 16 c.c) B30 =a cen ar 

4 NaOH = about o-o1N NaOH. 

(ee oe Fr » +16c.c. H,0+4c.c. N NaOH 
=about o-IN NaOH. 


Mix the solutions thoroughly and allow them to stand 
for 24 hours. Determine rate of flow, or better still, 
determine the age curves in the manner described above 
(EXDLEOS8): 

The addition of sulphates to a gelatin solution increases 
its viscosity considerably compared with that of pure 
gelatin. The iodides and bromides greatly decrease the 
viscosity. Carbonates, phosphates, oxalates, acetates 
and citrates raise the viscosity. The cyanides and thio- 
cyanates lower it. Chlorides, nitrates, and chlorates 
form complex changes, in so far as they can raise or lower 
the viscosity according to their concentration and the 
age of the gelatin. 

Additions of acids and bases, in the small concentra- 
tions mentioned, increase the viscosity. Greater con- 
centrations lower it again. A viscosimeter having a rate 
of flow for water equal to 150 fifth-seconds, gives the 
following values : 700 for a 0:5 per cent. gelatin solution ; 
3-4,000 for 0-005N HCl ; 300 for o-rN HCl; 2-3,000 
for 0‘o1 N NaOH; 500 for o-1 NNaQOH, etc. Prepare 
the complete concentration curves for HCl, NaOH and 
NaCl, and determine the viscosity 24 hours after pre- 
paration of the mixtures. 


VISCOSIMETRY OF CHANGESSOR 33 
AGGREGATION 


Expt. 72. Viscosity measurements on the coagu- 
lation of aluminium hydroxide hydrosoi—Measure 
the viscosity of a highly concentrated Al(OH), sol pre- 
pared according to Expts. 30 and 34. The viscosity of 


Sere TENSION AND. VISCOSITY 43 


this sol is higher than that of water, hence use a vis- 
cosimeter of wider capillary bore. Mix 8 c.c. of the sol 
with 2 c.c. of 2N KCl! and measure the changes in vis- 
cosity with time. Such an experiment gave the following 
results : 
Original Value 393 fifth-seconds 
After 15 minutes 406 


ay, 


”» 55 ” 417 5) 
reo = 3 427 3 
i Ba ae 445 :, 


Coagulation depends upon the concentration of the 
sol and that of the added solutions. After coagulation, 
the viscosity of the system again decreases and after 16 
hours the vigorously stirred solution gives a rate of flow 
equal to 415 fifth- 
seconds. tere ee: 

Expt. 73. Viscos- . fs 
ity measurements 
of the setting of 
plaster of Paris 
(Wo. Ostwald and P. ‘ ik 

Wolski, Kolloid. Z., hes 
27, 78 (1921)—Pre- te 

Patewae 5 per cent, 
suspension of finely 
powdered gypsum 
and transfer at once 
to a_ viscosimeter. 
Sedimentation may 
be prevented by 
drawing a continuous 


Time of Flore ———» 





Fa 


: Se, B 2555 30) SSG OS 
current of airthrough STMT eles 


the suspension. Ob- Fi: 5. 


1 Use an empirically determined KCl concentration which wil 
produce no flocculation within a half-hour, 


44 PRACTICAL COLLOID CHEMISia 


serve that the viscosity increases with the time as shown 
by Fig. 5. The process of setting may thus be followed 
by viscosity measurements. A viscosimeter having a 
diameter of 0-7-I-0 mm. and a water value of I00—150 
fifth-seconds is suitable for a charge of 20 c.c. 

Expt. 74. Viscosimetry of the formation of potato 
starch paste—Suspensoid systems, such as_ coarse 
suspensions of starch in cold water, show relatively small 
increases in viscosity, approximately proportional to their 
concentrations. On the other hand, hydrophile emulsoids 
show very great increases in viscosity both absolutely and 
relatively, with increases in concentration. In the forma- 
tion of potato starch paste, which, as is well known, 
generally takes place between 55° and 65° C.,a suspensoid 
system is converted into an emulsoid one. On warming 
the starch suspension, the viscosity decreases in accord- 
ance with the known decrease in the viscosity of water, 
the dispersion medium. When the temperature is raised 
between 55°—65° C. this decrease is, however, replaced by 
a great increase in viscosity, the most striking criterion of 
the formation of paste and of a radical change in the 
starch-water system. 

In order to measure the viscosity of a dispersoid which 
coalesces spontaneously after the fashion of unheated 
starch suspensions, a viscosimeter must be used whose 
rate of flow is relatively large compared to its rate 
of sedimentation.! Suitable viscosimeters have small 
volumes and short narrow capillaries or larger volumes 
and longer capillaries of wider bore. A 0-5—I-0 per cent. 
starch suspension may be used in the first type of viscosi- 
meter, whereas a 5 to 10 per cent. starch suspension is 
suitable for the second type. 

Warm the starch suspension in the viscosimeter in a 
water-bath with glass walls (a large beaker). Place the 

1 See Wo. Ostwald and H. Luers, Kolloid. Z., 25, 82, 116 (1919). 


pUnPaACee TENSION AND VISCOSITY 45 


thermometer in the liquid contained in the wide side arm 
of the U-tube, for the temperature of the suspension lags 
behind that of the water-bath. Warm the starch suspen- 
sion rapidly to 50° C., measuring the rate of flow every 
10°. Above 55° C. warm the suspension slowly at a rate 
of about 1° C. per 5 minutes. Lower the flame, measure 
the rate of flow continually and record the temperature of 
the water-bath after stirring. Stir the starch suspension 
thoroughly, by bubbling air through it before each 
measurement. If the concentration and the capillary 
bore of the viscosimeter have been suitably chosen, we 
find within a narrow temperature interval a change from 
decrease of viscosity to an increase as given in the follow- 
ing example : 

Five fer cent. suspension of commercial potato meal ; 
small viscosimeter, water value about 365 fifth-seconds at 
ay sie Cnarge, 10: C.c. 





Temperature. Time of Flow (T). Log. 1. 








See 239 2°30173 
ah: 228 rahoy pcs! 
56-2 226 *35411 
56°60 225 *35218 
DiS e274 ie Pe hers) 
“veg ae "34439 
58-1 220 °34242 
58:4 220 °34635 
58:8 228 "35793 
59°4 235 ops 
59°5 240 -38021 
59°7 244 "38739 








Between 58-1° and 58-4°C. there is a reversal of the 
viscosity change, i.e. the formation of starch paste. A 
graphical representation of the formation of starch paste 


46 PRACTICAL COLEOID CHEMISiE 


is given below in another experiment. If the viscosi- 
meter is not sensitive or the concentration of the starch 
suspension too small, the time of flow neither decreases 
nor increases, but remains practically constant within a 
range of 10°C. as in the following experiment : 

A 5 fer cent. suspension of potato meal; viscosimeter 
too large, water value about 150 fifth-seconds at 25° C.; 
charge, 20 €.C. 


Temperate soe 4I. 49 53 55 57° 59 Oba Ojo eos 
Time of Flow. . 112 106 103 101. 100,300) 100siQ0 =a (se 


For a more accurate determination of the temperature 
of starch paste formation, it 1s necessary to select a 
suitable viscosimeter so that the point of inflexion is 
observed within a very narrow temperature range. 

Furthermore, it is observed that in going above the 
temperature of starch paste formation, the times of flow 
determined in rapid succession are no longer constant but 
increase spontaneously because starch paste formation 
requires a certain amount of time. Such an observation 
can of course be utilized as an approximate indicator 
of the temperature of starch paste formation by deter- 
mining at which temperatures there are definite increases 
in viscosity according to three consecutive measurements 
made within 10 minutes. The use of a sensitive viscosi- 
meter is of course more accurate and more rapid.! 

To determine the exact temperature of starch paste 
formation, graphic representation of the data may be 
made in the following way: Plot the temperatures as 
abscissee and the logarithms of the times of flow as 
ordinates. From the data in the above table are obtained 


1 Such experiments demonstrate that a definite temperature of 
starch paste formation in a physico-chemical sense is a practical 
entity. Strictly speaking, there is probably a temperature range 
in which the rate of starch paste formation is abnormally rapid. 


— 


Peewee LE NSION AND VISCOSITY 47 


two practically straight lines (Fig. 6), which produced, 
intersect at a point. This experiment gives graphically 
a temperature of starch paste formation between 58:-2°— 
58:3° C. Ifthe graph of data obtained shows a horizontal 
line connecting the two oblique branches, it is due to the 
use of an unsuitable viscosimeter or to too small a con- 
centration of starch suspension. In such a case the two 
lines may be produced until they intersect at a point which 
would give an approximate 
value of the temperature of 
starch paste formation. SB 
Expt. 75. Viscosity mea- 
surements of the ageing of 
starch paste (M. Samec). ~* 
—Prepare al percent. starch 
solution by moistening 4 g. of 
potato starch with a small “sss 
amount of water and add 
gradually with constant stir- 
ring, 200 c.c. of warm but not hot water. Dilute to 
twice the volume, boil on a sand-bath for 30 minutes and 
after cooling make the volume up to 400 c.c._ Filter and 
cover with toluene for protection against bacterial action. 
Determine the time of flow for this suspension immediately 
after preparation. Measure the rate of flow of this 
suspension every day and plot the rates of flow against 
age. On plotting the viscosities obtained, the curve 
shows a large decrease in viscosity at first and gradually 
becomes asymptotic after an ageing of one to two weeks. 
Expt. 76. Viscosity measurements on the coagula- 
tion temperature of an albumin solution—Separate 
the yolk of a fresh egg from the white. Beat the latter 
into a foam and allow to stand overnight. The greater 


039) 


Zoe * 


59 60° 


5? 58 
Temperotur —> 


HiG,.6; 


1 Another example is given by Wo. Ostwald, Koll. Z., 12, 215 
(1913). 


48 PRACTICAL COLLOID CHEMEStiaa= 


part of the foam clears while the egg membrane remains 
suspended in the remainder of the foam. One egg gives 
about 20 c.c. of liquid, which is a mixture of albumin, and 
globulin. Dilute to twice the volume with a weak solution 
of 0-7 per cent. NaCl. The solution becomes turbid upon 
dilution with distilled water due to the relative insolu- 
bility of the globulin. Measure the change in viscosity 
with rise in temperature under the same conditions as 
those of the starch paste in Expt. 74, using the same rates 
of temperature increase. The time of flow decreases at 
first with rise in temperature until about 60° C. is reached, 
then either an increase or no change in time of flow occurs, 
depending upon the concentration of the solution and the 
sensitivity of the viscosimeter. The time of flow decreases 
again when the temperature exceeds 70° C. Observe that 














Temp. T. for egg white. T. for water. A 
51-0 yas 233 78 
55°0 292 220 W2 
56:3 286 217 69 
57:0 283 215 68 
57°60 281 212 68 
58°3 255) 212 65 
59:2 275 210 65 
60°3 | 270 206 64 
61-0 267 205 62 
61°6 268 204 64 
62:6 267 202 65 
62°8 207; 200 67 
63°5 266 199 67 
0357, 267 198 69 
64°5 266 196 70 
66-0 252 192 60 
66:9 247 190 57 
69°5 238 185 53 











a et. 


Senet TENSION “AND VISCOSITY 4Q 


the solution becomes turbid during the increase in the 
times of flow. 

The viscosity change during coagulation may be made 
more obvious by plotting the viscosity increase (i.e. time 
of flow of solution minus time of flow of H.O at the corre- 
sponding temperature) instead of the rate of flow as 
observed. This may be done by plotting a temperature— 
viscosity curve for water in the same viscosimeter. Com- 
pare the rates of flow of water with those of the egg white 
solution at the corresponding temperatures and plot in 
the same manner. A simple illustration of such an experi- 
ment is given graphically in Fig. 7,1 wherein the differ- 





60 
Temperatur —> 


BiG.7- 


ences in viscosities between albumin and water are 
plotted for increasing temperatures. The experiment was 
performed with equal volumes of egg albumin and 0-7 
per cent. NaCl solution. The viscosimeter was of the 
small type with a water-value of 365 fifth-seconds at 
250. 

The curve and data show that the coagulation of the 
albumin solution takes place at about 61°C. By plotting 


1 Another example is given by Wo. Ostwald, Koll. Z., 12, 214 
(1913). 
4 


50 PRACTICAL COLLOID CHEMISTie 


the legarithm of the rate of flow against the temperature, 
two approximately straight lines are obtained which 
intersect to give an acute angle. The determination of 
coagulation temperatures is practically conclusive only 
when all experiments are conducted under similar con- 
ditions. The viscosimetric method may be used to study 
the kinetics of effects of added salts, acids and bases on 
the course and mechanism of coagulation at the critical 
temperature range. 


1 More specific data is given in the monograph by H. Chick and 
C. T. Martin, Kolloidchem. Bethefte, 5, 49 (1913). 


IV 
OPTICAL PROPERTIES 
Ele AaAl, HETEROGENEITY 


r NHE optical heterogeneity of solutions is shown 
macroscopically by turbidity, microscopically 
by the so-called ultramicroscopic phenomena. 

Turbidity is best observed by contrast against a dark 
background. Better results may be obtained by placing 
a source of intense illumination to one side of the dark 
background or by holding the test-tube in a narrow beam 
of sunlight or a beam from a projection lamp. For 
ordinary purposes wrap a piece of black paper with a 
small hole in it around an incandescent lamp. The 
light from the source of illumination should not fall 
directly on the eye of the observer. 

For semi-quantitative purposes, prepare a comparison 
scale with milk or mastic hydrosol.!. Start with a con- 
centrated milk-white sol and dilute to give the required 
turbidity. A mastic sol is remarkedly stable for deter- 
minations of the turbidity of colourless sols provided an 
aged sol is used. Nephelometers, Tyndallometers ? and 


1 Diluted milk is especially suitable as a standard since the 
measurements of N. Manz (Dissertation, Marburg, 1885) showed 
that it absorbs all wave-lengths equally. F. B. Young [PAzl. 
Mag. [6] 20] 793 (1910) | used diluted milk as a standard for degrees 
of turbidity of ether at the critical temperature. 

*See B. H. von Oettingen, Z. f. physik. chem., 33, I (1900) ; 
J. Friedlander, ibid., 38, 385, 413 (1900) ; C. Benedicks, Koll. Z., 

51 


52 PRACTICAL COLLOID CHEMISTRY 


also colorimeters may be used for more accurate deter- 
minations of turbidity,! which is the ratio of the light 
diffracted by the colloid particles to that transmitted. 

Expt. 77. Detection of faint turbidity by means 
of the Faraday-Tyndall light cone—A large number 
of disperse systems appear completely transparent upon 
superficial examination, especially in transmitted light. 
However, they produce a decided Tyndall effect. 
Examine the following sols by transmitted light and then 
by a narrow beam of light from a projection lantern : 
a red gold sol, freshly dissolved collargol, freshly prepared 
arsenic trisulphide or Prussian blue, as well as a I per 
cent. solution of potato starch paste which has been 
heated for thirty minutes. 

A cold saturated solution of cane sugar gives a bluish- 
white light cone. 

Expt. 78. Polarization of the Tyndall light cone— 
Place a turbid colloidal solution as mastic hydrosol in 
the beam of a projection lantern. Use a sol of such 
dilution that it will give a well-defined Tyndall cone 
unaffected by too an intense source of light. Examine 
this cone with a Nicol prism at right angles to the beam of 
light. The cone of light disappears or becomes dim twice 


7, 204 (1910) ; Th. W. Richards, Proc. Amer. Ac., 30, 385 (1904) ; 
Am. Chem. J., 31, 235 (1914); 35, 510 (00@G) ee ee 
ibid., 35, 100 (1906); E. Schlesinger, Berl. Klin. Wochenschr., 
48, 42 (1911), etc.; accurate measurements are given by W. 
Steubing, ‘‘ On the optical properties of colloidal gold solutions,” 
Dissert., Greifswald (1908); Ann. d. Physik, 26, 329 (1909) ; 
W. H. Keésom, Ann. d. Physik, 35, 501 (19011) ;) Comma evs. 
Tab., Leyden, No. 104 (1910) ; W. Mecklenburg and S. Valentiner, 
Z. f. Insirumenthunde, 34, 209 (1914) ; Kolloid. Z., 15, 99 (1914) ; 
16, 97 (1915); F. Sekera, Koll. Z., 27,728 (1021) . hee 
ibid., 27, 236 (1921). 

1 An apparatus like the Wilh. Ostwald-Donnan’s Colorimeter 
is suitable (Physico-chemical Manual, 3rd Ed., 358 (1910). 


OPTICAL PROPERTIES 53 


in one complete revolution of the prism. The light is thus 
partially polarized. Perform similar experiments with a 
very dilute solution of a fluorescent substance, such as 
quinine sulphate, alkaline fluorescin or eosin and observe 
that no dimming of the light cone occurs by revolving the 
prisms. Fluorescent light in distinction from a light ray 
of a turbid solution, is not polarized. 

Expt. 79. Turbidity and degree of dispersion— 
Experiment and theory show that turbidity is greatest in 
a solution of moderate concentration of the disperse 
phase. Therefore, the maximum degree of turbidity does 
not occur in colloids, but in coarsely disperse systems. 

Place freshly prepared mastic, arsenic trisulphide or red 
gold sol in two beakers and to one add a few drops of 
HCl or BaCl, solution. Compare the turbidities by 
means of a Tyndall cone and observe that the coalescing 
sol shows a considerably greater turbidity. 

Perform the same experiments with dilute ‘‘ Congo 
rubin.”’ The “ pure’’ solution seldom shows a light cone 
in a nephelometer. Add a few drops of an electrolyte as 
HCl, Ba(OH),, until the solution gradually changes to a 
blue-violet colour as the turbidity increases. The 
coagulated sols show individual coarsely disperse particles 
when shaken. The diffracted light which these coagu- 
lated particles radiate is less intense than during the 
initial stages of flocculation. 


TURBIDITY PHENOMENA IN HYDROPHILE 
COELOIDS 


Changes in turbidity are not only dependent on the 
variation in the degree of dispersion but also on the 
degree of hydration. Every emergent ray from a colloid 
solution whether produced by refraction or reflection, is 
due to a distinct variation in the optical relations between. 


54 PRACTICAL COLLOID CHEM 


the disperse phase and the dispersion medium. This 
difference is less the more hydrated is the disperse phase. 
The difference increases if dehydration of the disperse 
phase takes place. Variations in the degree of dispersion 
and hydration frequently occur at the same time. 
Marked variation in the turbidity of a colloid solution may 
occur with but slight changes in external conditions. 

Expt. 80. Changes in turbidity of aqueous gelatin 
solutions with concentration—Prepare a_ series of 
gelatin solutions! of the following concentrations by 
diluting with warm water : 


65 4 -30 27°35) 50. 30 Sper 


After the solutions have cooled allow to stand over- 
night in an ice-box. The maximum turbidity occurs not 
in the most concentrated solution but in the one of 
medium concentration such as 2-3 per cent. Gelatin 
solutions prepared at a lower temperature show the 
turbidity maximum more distinctly. 

Soak a thick sheet of gelatin or a transparent sheet of 
glue in a beaker filled with water. After a few hours, 
compare the soaked swollen portions with those still 
unaffected. Observe that a considerable increase in 
turbidity has occurred in the portion swollen by the water. 
Dry a piece of 30 per cent. gelatin jelly in an oven or ina 
desiccator over H,SO,. Do not use too high a temperature 
when drying with heat on account of the tendency 
of the gel to liquefy. The 30 per cent. gelatin jelly is 
very turbid, but it becomes less so with gradual loss of 


1 The gelatin is purified by washing for 2-3 days with continu- 
ously flowing water or by frequently replacing with distilled water, 
and the weight of the gelatin determined before and after swelling 
with the added precaution that none of the gelatin is lost during 
the washing... The latter may be realized, in the author’s experi- 
ence, by using boiled porous sacks. The experiment may at times 
be carried out with unwashed gelatin. 


OPTICAL PROPERTIES 55 


water and finally almost transparent when the original 
thickness of the gelatin is attained. Therefore a gelatin- 
water mixture of various concentrations may have two 
degrees of maximum turbidity. 

Expt. 81. Effect of dehydration on turbidity of 
silicic acid gels—Prepare according to Expt. 25 a clear 
aqueous solution of silicic acid by mixing two parts of 
2N acetic acid with one part of water, and after cooling 


the mixture, carefully add one part of Io per cent. water- - 


glass.1_ Place the greater portion of this gel in a desiccator 
over concentrated H,SO,. This dries in the course of I to 2 
weeks at a rate which may be determined by periodic 
weighings. Observe that with a water content of 35-55 
per cent., the apparently clear jelly containing some gas 
bubbles gradually becomes turbid. Generally, the centre 
of the jelly mass first shows an opalescence which gradu- 
ally extends in all directions. This illustrates the sudden 
“transitions ’”’ of the silicic acid gel. The turbidity dis- 
appears after longer desiccation and the jelly becomes 
as transparent and as firm as glass.2_ If a dried piece of 
jelly is placed in a flask with moistened filter paper and 
the flask sealed with a stopper, the hardened gel frequently 
disintegrates with a noise as a result of internal stress. 
Such gels usually show a new “ transition ”’ after rehydra- 
tion but not to as marked a degree. : 

Expt. 82. Gelation and _ turbidity—Allow one 
portion of a 2-3 per cent. gelatin solution to solidify at 
room temperature, another portion in an ice-chest and 


1 Acetic acid yields as a rule clearer gels than HCl. 

2 Good results have not always been obtained by the author, 
as J. M. van Bemmelen has already reported. Occasionally the 
author has been able to observe the phenomenon on drying the 
silica gel, previously washed with HCl, in the air and also warming 
it gently. Sometimes, in spite of apparently similar conditions, 
the phenomenon has not been observed ; however, it may have 
been missed during the night. 


56 PRACTICAL. GOELOID, CHEMisi a 


keep the remainder liquid at about 30°-40° C. The solution 
which solidifies most rapidly at a low temperature is the 
most turbid and that solidifying at room temperature is 
more turbid than the fluid solution at an elevated tempera- 
ture. 

Fill two beakers with the same 2-3 per cent. gelatin 
solution, liquefy both portions by placing them in hot 
water for about 30 minutes. This liquid jelly is much 
more turbid than the solid portion. 

Expt. 83. Ageing phenomena and _ turbidity— 
Let a I per cent. solution of starch paste age in an ice- 
chest (Expt. 75). The freshly prepared solution shows 
a bluish-white opalescence. Its turbidity increases con- 
siderably upon ageing and in the course of 2—3 weeks it 
becomes white and opaque. A considerable increase in 
turbidity of the jelly is observed after 24 hours. 

Expt. 84. Influence of electrolytes upon the 
turbidity of gelatin jellies—Prepare about 150 c.c. of 
2-3 per cent. gelatin solution. This concentration of 
jelly shows the maximum turbidity (Expt. 80). Pour 
ro c.c. of this solution into a number of test-tubes and add 
to successive portions a drop of 2N solutions of NaOH, KI, 
KNCS, KCl and Na,SO, so that a o-oIN solution results. 
Place these tubes in the ice-box, as well as two control 
tubes which contain no additions. The following series of 
decreasing turbidities are observed after 1 to 2 days: 


Control, KI, KNCS, KCl, Na,SO], Gis siege 


All electrolytes in the concentrations given above pro- 
duce a decrease in the turbidity of the jelly. Acids and 
bases exert the strongest effect. 

Expt. 85. Critical turbidity—A distinct turbidity 
maximum is observed at an intermediate stage of mixing 
two liquids soluble in one another to a limited extent 
(J. Friedlander, V. Rothmund). Prepare a mixture of 


OPTICATS “PROPERTIES 57 


about 36 parts of colourless solid phenol and 64 parts of 
water. At room temperature, there are two layers, which 
form molecular disperse immiscible systems. Heat the 
mixture to 70° C. and shake continuously until the turbid 
emulsion becomes clear. Continue to shake the solution 
and allow it to cool slowly. At first a very slight tur- 
bidity occurs which appears as a colour phenomenon or 
as an opalescence. This is indicative of the emulsoid 
state.! On further cooling the turbidity increases con- 
siderably and a coarsely disperse emulsion appears which 
gradually separates into two distinct layers. 


ULTRAMICROSCOPY 


According to the theory of microscopy, particles appear- 
ing geometrically similar are greater than the wave-length 
of hight to which they are exposed. Such particles may 
be differentiated from one another provided they are 
spaced at intervals greater than one-half the wave-length 
of the hght by which they are illuminated. The smaller 
colloidal particles have the dimensions of about o-Im, and 
hence cannot be optically distinguished with an ordinary 
microscope. However, it is possible to distinguish single 
colloidal particles without their individual geometric 
forms by means of an intense lateral illumination and not 
by transmitted light. By this method, particles which are 
considerably smaller than the wave-length of light may be 
recognized individually, for they reflect the light in all 
directions and are consequently self-illuminative. 

An ultramicroscope consists of an intense Faraday- 
Tyndall cone which strikes a microscope with a special 
. attachment to make colloid particles visible. 

Water for ultramicroscopy—Water with the least 


1See Wo. Ostwald, The World of Neglected Dimensions, 8th 
edition (Dresden, 1922), p. 72. 


t 


58 PRACTICAL’ COLTOID: CHEMiSai 


amount of optical impurities is obtained by storing a 
large volume of distilled water at a uniform temperature 
for a long period of time and then siphoning off the upper 
portion of the water. Glass and hard rubber stoppers 
give ‘‘ optical dust ’’ which may be prevented by covering 
the stoppers with paraffin or tinfoil. Ultrafilters im- 
prove the water considerably, especially if the water is 
carefully excluded from the air after filtration. The 
number of dust particles present is usually small and the 
experimenter soon learns to recognize their presence. 

Expt. 86. Suspensoids—Ultramicroscopic experi- 
ments may be easily performed on the separate particles 
of suspensoid colloids, if the distance between each particle 
is relatively great. The colloidal solution must be very 
dilute so that the particles are at sufficient distances from 
each other, to obviate mutual reflection phenomena which 
would blur the particles when viewed individually against 
a dark background. 

Mastic hydrosol—A very dilute sol prepared accord- 
ing to Expt. 1 shows a large number of white, intensely 
illuminated particles in rapid Brownian movement. The 
interference of aggregated particles is certainly appreciated 
if observations are made after alternate additions of the 
concentrated solution and distilled water. A dark back- 
ground is necessary for the easy detection of the individual 
particles. 

Black India ink gives an image similar to mastic sol 
when using a much greater dilution. The background 
cannot be made so dark because of the presence of 
hydrated or protective colloids. Both the black and 
colourless particles reflect white light. 

Gold hydrosol—Observe and compare ultramicro- 
scopically the red and blue gold sols prepared according to 
Expts. 2-7. Smaller particles are usually found in the 
red sol rather than in the blue sol unless the red sol is 


Piety 


PEUIGAL PROPERTIES 59 


prepared in the presence of protective colloids. Red sols 
occasionally show larger particles, which probably are 
soluble aggregates of smaller particles. A highly disperse 
dilute gold sol cannot be further resolved ultramicro- 
scopically, but gives only a diffused light cone. 

Other experiments on suspensoids—Colloidal silver 
and other metal sols! ; Prussian blue, metallic sulphides, 
organic dyes such as indigo, alkali blue, alizarin in paste 
form are suspensoids which are suitable for ultramicro- 
scopic experiments. 

Quantitative studies of the dimensions of particles 
cannot be based upon the size and intensity of the 
illuminated spots observed. For experiments’ on 
approximate determinations of particle size, see H. 
Siedenkopf and R. Zsigmondy, Ann. d. Phys., 10, 16 
(1903) ; G. Wiegner, Koll. Beth., 2, 213 (1911). Particles 
of miscroscopic dimensions, those of 0-2” and more in 
diameter, are recognizable by the formation of refraction 
figures, such as concentric circles, V- or Y-shaped light 
haloes and other complicated light figures. Typical 
colloidal suspensoid particles produce comparatively 
bright and approximately circular light areas. 

Emulsoids—Non-hydrated emulsoids such as_ oil- 
water sols (Expt. 1) give the same ultra-images as suspen- 
soids. The difference in optical constants between the 
disperse phase and dispersion medium, which are neces- 
sary for ultramicroscopic recognition of the particles, 
disappears -with increasing hydration. The undiffer- 
entiated light cones always become dimmer with increas- 
ing hydration of the suspensoid and may practically dis- 
appear. Such negative results are due to smaller particles 
in the emulsoid previous to hydration. One must dis- 
tinguish between an optical amicroscopy and a dimen- 
sional amicrescopy. A negative ultramicroscopic obser- 

1 Compare the preparations given in Chapter IX. 


69 PRACTICAL COLEOID CHER ai 


vation does not prove the presence of a highly disperse 
emulsoid. Many solutions may be recognized as colloid 
systems in consequence of diffusion, dialysis, ultra- 
filtration, etc., yet they give only a negative or diffuse 
ultramicroscopic image. Starch paste is a good example 
of such an emulsoid. 

Ferric hydroxide sols—These sols are transitions 
between suspensoids and emulsoids, and illustrate emul- 
soid properties even better than solutions of egg white, 
gelatin, etc. Ferric hydroxide does not contain so many 
coarse impurities as the viscous albumin and gelatin sols. 
Sols which prove to be typical colloids by dialysis or 
ultrafiltration show at first an increase in light intensity, 
but on further dilution they show a light cone not resolv- 
able into single particles. The commercial Fe(OH), or 
preferably that prepared in Expts. 21 and 22, is suitable 
for these experiments. 

Solutions of egg white, gelatin, silicic acid, stannic 
acid, etc., usually show a diffuse cone containing much 
“optical dust.’”’1 A 0-5-I-0 per cent. solution of potato 
starch which has been heated at 100° C. for 30 minutes 
appears relatively clear. Starch paste shows a consider- 
ably diffuse light cone, in which only few “ dust ’’ particles 
are imbedded. If the starch sols are heated at 100° C. 
for about Io minutes, they coagulate and show a greyish- 
white irresolvable light cone. Nevertheless, the starch 
particles are not so coarse to settle on standing. This is 
an example of the above-mentioned fallacy of assuming 
dimensional amicroscopy from optically clear images given 
by sols whose particles are irresolvable. In previous 
experiments the starch particles were apparently so 


1 Optical and chemical purity are not necessarily equivalent. 
Sodium hydroxide may be prepared with the greatest precaution 
from fresh metallic sodium and yet may not be optically as clear 
as an old caustic solution whose impurities have settled. 


OPTICAL PROPERTIES 61 


strongly hydrated, especially in their external layers, and 
were so thickly aggregated that the optical transition 
between disperse phase and dispersion medium was 
practically constant. Dyes such as safronine, night blue, 
etc., usually contain so many impurities that they tend 
to destroy the typical image of an irresolvable light cone. 

Freshly prepared, very dilute silicic acid and solutions 
which contain great excess of acid appear optically clear. 
This also applies to serum and egg albumin solutions to 
which a few drops of HCl or NaOH have been added. 


ViiaoonOsCOPIC CHANGES OF STATE 


The large number of experiments on_ turbidity 
phenomena described in the previous paragraphs (77-85) 
may also be performed ultramicroscopically by observing 
changes of state in colloidal solutions. 

Expt. 87. Ultramicroscopy of gelation—According 
to the experiences of the author a solution of “ pure”’ 
gelatin is essential for an accurate study of the changes 
of state. This solution is best prepared by washing a 
2-3 per cent. gelatin for several days. 

This lukewarm ! solution shows a grey-white Tyndall 
cone containing many impurities and showing Brownian 
movement. Choose an ultramicroscope bulb with a 
clesed stopcock, wash with alcohol to prevent the cloud- 
ing of the window, fill it with gelatin solution and place 
in an ice-chest for 24 hours. The ultramicroscopic imege 
first shows a considerably increased illumination of the 
whole light cone; later, a great number of light rays 
appear, which orient themselves to form a coarse structure 
in an apparently regular manner. 


1 At higher temperatures the sealing wax which binds the cover- 
glasses to the cuvette melts. 


62 PRACTICAL, COELOID CHEMTSii 


For a control experiment,! liquefy the gel by moistening 
the bulb with water at 40°-50° C. for a few minutes. 
Observe the considerable decrease in the intensity of the 
light cone, the disappearance of the Brownian movement. 
These effects may also be observed when the gelatin 
solution is heated to a higher temperature in order to 
completely disintegrate the coarse aggregates formed by 
gelation. By studying the process of gelation with a 
strong lighting apparatus, one may observe the gradual 
transitions from Brownian movement to oscillatory 
motion to complete immobility of the particles, and finally, 
to the formation of larger aggregates (W. Menz, W. 
Bachmann). 

Expt. 88. Ultramicroscopy of the ageing of starch 
pastes—Ultramicrcscopic observations may be made 
together with viscosimetric experiments on the ageing 
of a I per cent. starch paste (Expt. 75) so as to co-ordinate 
changes in turbidity with the variations in its viscosity. 
The presence of particles possessing Brownian movement 
is especially evident with the cccurrence of complexes in 
aged starch sols. Brownian movement is absent or only 
very slight in cold freshly prepared starch pastes. 

Expt. 89. Ultramicroscopy during coalescence— 
Fill the ultramicroscope with a very dilute mastic hydrosol 
or black India ink. Copy the image and count the 
number of particles in a portion of the field bounded by an 
ocular grating. Use the centre of the optical field for the 
observations. Add to the bulb about two drops of HCl or 
BaCl, and thoroughly mix the contents by pouring the 
whole solution into a small beaker and back again into 
the bulb. After standing a few minutes there is a decrease 
in the Brownian movement with the formation of larger 

1 The cuvette should be carefully washed after these experi- 


ments by means of tepid warm solutions of KI, KCN or KCNS, 
which are good solvents of gelatin. 


OPTICAL PROPERTIES 63 


irregular aggregates. Another count shows a decrease in 
the total number of particles as a result of aggregation. 
Similar experiments with red gold sols show a change of 
colour when the particles aggregate and hence decrease 
in number (Expt. 60). The coagulation of dilute ferric 
hydroxide sols with a drop of NaOH is striking. Slightly 
disperse granular particles are seen to coalesce in groups 
which become quite distinct from one another and finally 
unite to form very large flakes. A similar coagulation 
process may be observed by flocculating 0-or per cent. 
sols of Congo rubin and antimony sulphide (Expt. 13). 


Toe ONS Or THE PLANE OF POLARIZED 
PiGat BY COLLOIDS 


Hydrated colloids such as egg white, gelatin, tannin, 
starch paste, etc., strongly rotate the plane of polarized 
light. This phenomenon is very interesting and yet has 
been little investigated. An ordinary saccharimeter with 
a sodium flame, intensely illuminated to overcome the 
turbidity of the solution, is suitable for observing this 
phenomenon. 

Expt 90. Optical rotation by gelatin solutions (H. 
Trunkel)—The degree of optical rotation of a gelatin 
solution is as variable a property as its viscosity. The 
degree of rotation increases with increasing concentra- 
tion, decreasing temperature and is also dependent upon 
the age of the gelatin solution. The effect of age upon 
optical rotation may be determined as follows: Fill a 
200 mm. polarimeter tube with a freshly prepared gelatin 
solution of a concentration such that the intensity of the 
light source may suffice. The experiment should be per- 
formed at constant temperature especially if the polari- 
meter 1s provided with a water-jacket. Record the degrees 
of optical rotation every hour and observe the constant 


64 PRACTICAL ‘COLLOID CHEMIST 


increase at high concentrations and low temperatures. 
The degree of rotation generally reaches a maximum after 
2-4 days. 

The following results were obtained at room tempera- 
ture by using a clear 5 per cent. solution of gelatin, a 
200 mm. polarimeter tube and sodium flame :— 





t a Rotation. A 








Time in Hours. | Observed Degree. | Calculated Degree.| 12 Per cent. 
0°47 7:97 7°97 0-0 
A hy (eer 7°94 oa 
2°48 9°72 7°49 + 0°23 
Bez IO'l5 10°09 -+ 0:06 
4°47 10°27 10°42 — o13 
5°55 10°63 10°79 — O16 














The calculations conform to the equation a= K¢?”, 
in which ¢ is the time and K and m are constants. 

Expt. 91. Optical properties of vanadium pent- 
oxide sols (H. Diesselhorst and H. Freundlich)—The 
freshly prepared sol, Expt. 33, does not appear to have 
optical properties. A vanadium pentoxide sol, after 
standing a few weeks, when stirred with a glass rod, 
shows silky streaks. These streaks are yellow in reflected 
light and dark in transmitted light. The sol, which can 
be diluted to give a bright brown colour in transmitted 
light, is placed in a cuvette between two crossed Nicol 
prisms. On stirring, a brightening of the field of vision 
is observed, after which dark clouds reappear. A sol 
kept for six months is so sensitive that it glows on shght 
stirring. This is the characteristic behaviour of a fluid 
crystal. 


SEICAL PROPERTIES 65 


COLOUR OF COLLOID SOLUTIONS 


The colour phenomena in colloid solutions are brought 
about in two ways. Colloid particles show a selective 
absorption of light rays possessing certain wave-lengths. 
Some of the colour phenomena are due to the small size 
of the particle, which radiates laterally a considerable 
amount of light. This type of radiation is selective and 
hence various colloids reflect different coloured light rays. 
These conditions of colloidal state account for the double 
colour phenomena often occurring in colloids, wherein 
the difference in colour depends whether the light is 
transmitted or reflected. Selective adsorption no doubt 
gives the colour to transmitted light, while selective 
radiation is responsible for the colour of reflected light. 
Often, the colour of the reflected light is complementary 
to the colour of the transmitted light. 

Selective adsorption and selective radiation, together 
with the degree of dispersion, the orientation as well as 
the shape and mass of the particles play a considerable 
rdle in the colour change. 


CeEOUnReOte COLOURLESS COLLOIDS ” 


Colourless substances are those which absorb ultra-red 
or ultra-violet light rays. If these substances are dis- 
persed in colourless dispersion media, they show the 
usual colour phenomenon of opalescence irrespective of 
their individual properties. These sols impart a yellow 
or red colour to reflected light. The best example of 
such an opalescence is the cloudy sky at sunrise and sun- 
set due to transmitted light and in the daylight due to 
light reflected from a dark background. This opalescence 
results from the retardation of the light of shorter wave- 
lengths by the colloid particles, that is, they retard the 


5 


66 PRACTICAL COLLOID, CHINES ian 


blue and violet rays more than the longer yellow and red 
rays. The yellow and red light rays pass through 
the colloid with the least amount of retardation by 
the particles, while the blue and violet rays are strongly 
refracted or radiated. Thus, hght is dispersed into 
its different wave-lengths so that the longer waves 
passing through produce absorption colours, while the 
shorter waves are deflected laterally, producing refraction 
colours. Opalescence may be distinguished from fluor- 
escence by the fact that opalescent light is, while fluor- 
escent light is not, polarized (Expt. 78). 

Expt. 92. Opalescent solutions—A beautiful opal- 
escent solution may be prepared in the following way: 
Pour about 50 c.c. of a o-1 per cent. alcoholic solution of 
mastic or colophonium into 200-300 c.c. of distilled 
water or prepare a sulphur sol according to Expt. II. 

Add in small portions 100-200 c.c. of boiling distilled 
water to about 50 c.c. of a 0-5-I-0 per cent. filtered solu- 
tion of dried egg albumin preserved in a 0-9 per cent. 
NaCl solution. To prepare a sol of fresh egg white, as 
in Expt. 103, beat the white to a foam and allow to stand. 
Separate the clear fluid from the membranous foam. 
Dilute this clear liquid with four times its volume of 
o-g per cent. NaCl solution and slowly add 300 c.c. of 
boiling water. 

Other beautiful colour phenomena obtained from 
colourless dispersoids are the so-called Christiansen 
diffraction colours observed in the NaCl gel prepared 
in Expt. 27. Furthermore, polymerized cinnamic acid 
ethyl esters ; many liquid crystals ; fine suspensions of 
glass, quartz, NaCl etc., in mixtures of organic solvents, 
which have almost the same coefficient of refraction, also 
show these colour phenomena. See B. C. Christiansen, 
Ann. d. Physik. 23,298 (1884) ; 24, 439 (1885). Asimple 
example of this phenomena is an aqueous saturated 


OPTICAL PROPERTIES 67 


solution of H.S, in which decomposition has begun. 
By holding the coalescing solution against the light, a 
distinct violet absorption colour may be observed. NaCl 
gel produces the two colours, yellow and bluish green ; 
and cinnamic ester the colours green and red at room 
temperature, and yellow and blue at higher temperatures. 


COLOUKS OF COLLOIDAL METALS 


Colloid metals show a great variation in colour 
phenomena. M. Faraday pointed out that the degree of 
dispersion is largely responsible for colour formation. 

Expt. 93. Polychromism of gold sols—Expts. 2-7 
give the methods of preparation of red, violet, blue and 
green gold sols. A simple experiment which successively 
produces all the gold sol colours mentioned is similar to 
the method described in Expt. 3, using alcohol and a 
reducing agent. Use a large volume, 100-150 c.c. of 
boiling water, and add in the manner described above, 
5-I0 c.c. of a o-or per cent. gold salt solution, and the 
same amount of alcohol. Warm the mixture until the 
red colour is developed. Pour a test portion of the hot 
sol into a small Erlenmeyer. Keep the main portion 
boiling continually and add, drop by drop, more gold 
salt solution, without any addition of alcohol. Continue 
to heat until a violet to blue sol appears and remove 
another test portion. To prepare a green sol, add 10-20 
c.c. more gold salt solution to the remaining 50—I00 c.c. 
of hot violet sol. Observe that in such a series of variously 
coloured gold sols, prepared from the same original 
solution, the increase in turbidity follows the order of 
colour change from red to green. 

Expt. 94. Polychromism of silver sols—A series 
of coloured silver sols ranging from bright yellow, through 
the various shadings of red to blue and bluish black, 


68 PRACTICAL COLLOID: CHEMISi 


is prepared in the following manner: First prepare 
three solutions: 0o-oIN AgNO;, 0-o001M hydroquinone 
[C,H,(OH).] and 0-o1N sodium citrate. Make the latter 
solution by titrating o-5N citric acid with an equal 
volume of 0:-5N NaOH until added phenolphthalein just 
assumes a pink tinge. All solutions, especially the AgNOs, 
should be neutral. Use freshly prepared hydroquinone 
solution. A very slight excess of alkali is necessary for 
reduction. Make the following preliminary experiments 
in order to standardize the hydroquinone solution. 
Add 2 c.c. of hydroquinone and 4 c.c. of sodium citrate 
to 2 c.c. of silver nitrate solution. If the mixture does 
not develop a faint yellow colour after Io seconds, add 
a drop of dilute NH,OH to 100 c.c. of the sodium citrate 
solution and repeat the experiment. Should the yellow 
colour not appear in 10 to 15 seconds, add two drops of 
NH,OH to the citrate solution and continue to do so 
until the desired reaction takes place. The presence of 
much NH,OH soon destroys the coloration by floccu- 
lating the silver sol. 

After standardization of the citrate solution, place 2 c.c. 
of the AgNO, solution into ten well-cleaned test-tubes, 
and add the following mixtures of hydroquinone and 
sodium citrate. 





Exp. No. I 2 iS 4 5 6 z 8 9 10 
































Hydro- 
quinone |5 drops.|7 drops,|10 drops.|14 drops.| I C.c.|1°4 ¢.c.| 2 ¢.c. |2°8c.c.| 4 C.c. | 5°6 C.c. 
Citrate .| 16. ¢:c. | r1¢.¢. | 8:c:c) | 5°6.¢.c. |4 C.C)2-8ie Cl aese em reac cee x4 drops 


If no reaction 1s evident in the first two or three tubes, 
a few more drops of NH,OH may be added to them with- 
out danger of flocculation. All the mixtures first assume 
a yellow or red colour, but they gradually develop a graded 
series of colours toward the blue of the last tube. After 


BEEiCAL PROPERTIES 69 


3 hours, the tubes have the following colours: 1, bright 
yellow; 2, yellow; 3, orange-yellow; 4, orange; 5, 
red-orange; 6, red; 7, red-violet; 8, violet; 9, blue- 
violet; 10, blue. The last sol must be diluted with 
water in order to make the colour more distinct. The 
sols remain stable a few days, when all the colours gradu- 
ally change to blue. 

Green sols cannot be prepared in the cold by this 
method. Pour the above mixtures all together, stir and 
heat to boiling. A yellow sol appears before the colours 
fade. If solution 10, which finally becomes blue, is 
heated at the point where it is faintly red, a greenish 
coloured sol forms. Continued heating produces a yellow 
sol and finally changes it toa blue one. By this method, 
approximately any shade of green between blue and 
yellow may be prepared. 

Observe the increase in turbidity in the series ranging 
from yellow to blue. The continuous change in colour 
from yellow to blue corresponds to a change in the ab- 
sorption maximum of the shorter to longer wave-lengths 
with a decreasing degree of dispersion. This is a general 
phenomenon in colloid chemistry illustrating the relation 
between colour and degree of dispersion. 

Expt. 95. Polychromism of sulphur sols (R. 
Auerbach)—Mix Io c.c. of 1-33 per cent. phosphoric acid 
(Io c.c. of commercial H;PO, and dilute to 150 c.c.) with 
Pomc oieO05 1 Naj>.0,. After a few minutes, the 
absorption colour becomes yellow; reflected colour, 
blue; then the absorption colour becomes red; the 
reflected colour very turbid and greyish white. Later a 
dark blue or occasionally green shade appears, and 
finally flocculates to give a white, coarsely disperse 
sulphur sol. 

Expt. 96. Colour changes in gold sols during 
flocculation—tThe relation between colours of sols and 


70 PRACTICAL “COLLOID CHEMIST 


the size of their particles is evident by the sudden change 
of red gold sol into a violet or blue upon addition of an 
electrolyte which produces flocculation. 

Place in large test-tubes or Erlenmeyer flasks equal 
volumes of a red gold sol and add to respective portions 
a drop of dilute HCl, NaCl, BaCl,, etc. After a few 
seconds, the red sol suddenly changes into a violet or 
blue sol. The occurrence of turbidity in the Tyndall 
cone, the ultramicroscopic image and the ultimate 
appearance of flocculation show that the gold sol forms 
greater complexes during the sudden colour transition. 
Reversible colour changes of colloidal gold in the presence 
of casein have been shown by R. Zsigmondy, Nachr. d. 
Gottinger Ges. d. Wiss., January, I9g16. 

The silver sol prepared in Expt. 94 shows the sudden 
change of colour upon flocculation with electrolytes. 
The colour of the silver sol after complete flocculation 
is usually black. 

Expt. 97. Colour changes in Congo-rubin sols— 
The particles of this dye sol have diameters between those 
of colloids and molecular dispersoids. It may be sud- 
denly transformed to a blue-violet or blue solution not 
only upon addition of acid but also by the addition of 
any neutral salt or even alkaline substances. The dye 
behaves like a red gold sol in many respects and it may 
be used as a gold sol substitute. The colour transition 
of Congo rubin is reversible by dilution, by raising the 
temperature, by addition of alcohol, etc. The colloidal 
changes in this dye are observed in the following ex- 
periment : 

Use a o-o1r per cent. solution of Congo rubin. Place 
10-20 c.c. of Congo-rubin solution in a large number of 
test-tubes and add to respective tubes a few drops of some 
common chemical reagents. All electrolytes cause colour 
transition except strong alkalies and NH,OH. 


OPTICAL PROPERTIES 71 


Warm the Congo-rubin solution, coloured blue-violet 
by a small amount of electrolyte, until the solution turns 
red. Pour half of this hot solution into a cold test-tube 
and cool further in a stream of water. The red solution 
turns blue once more, 

To a large beaker of distilled water add, drop by drop 
with constant stirring, a solution of Congo-rubin blue 
dye. After a few minutes the violet tinged solution 
changes to bright red. 

Add 2-3 c.c. of a blue Congo-rubin solution to an equal 
volume of methyl or ethyl alcohol. The red colour again 
appears with a simultaneous disappearance of turbidity. 

For further experiments with Congo rubin, see numbers 
159, 177 and 179. 

Expt. 98. Colour and degree of dispersion— 
From the preceding experiments, the relations between 
colour and degree of dispersion of variously coloured gold, 
silver and Congo-rubin sols have been illustrated. This 
relation is important especially in the theory of colours 
of substances in the colloid state. The following experi- 
ments demonstrate this relation : 

Ultrafilter according to Expt. 57, red, blue, green gold 
sols, yellow, red and blue silver sols, red and blue rubin 
solutions. The first members of these series pass un- 
changed through the 2-4 per cent. collodion filter. How- 
ever, the ultrafilter retains the blue and green sols, while 
the behaviour of the intermediate series varies. 

A mixed colour sol is changed by ultrafiltration, so 
that a sol of another and purer colour constitutes the 
ultrafiltrate. Adda few drops of dilute acid or neutral 
salt, baryta, etc., to a Congo-rubin solution, so that 
a violet shade just appears, and ultrafilter this solution. 
The filtrate consists of the red coloured sol. Pour 
together the different coloured silver sols obtained in 
Expt. 94 and ultrafilter the mixture. A highly disperse 


72 PRACTICAL COLLOID CHEMIST 


yellow silver sol is obtained as an ultrafiltrate from the 
dark grey mixture. 

The relation between colour tone, turbidity, and 
ultramicroscopic images of colloids was brought out in 
the above experiments. Compare the ultra-image of the 
red Congo-rubin solution, and the image of the blue 
solution, upon addition of electrolytes. 

Expt. 99. Ultramicroscopic colours—The colour 
of the particles visible in an ultramicroscope is due to 
the selective absorption, selective refraction and radiation. 
Lateral radiation colours of colloid particles may be 
observed microscopically in very dilute solutions. When 
using large quantities of concentrated solution, the colloid 
layer lying above the light cone acts as a light filter. 
The radiated colours of the single particles are altered 
by the absorption colours of the entire colloid. 

Concentrated ferric hydroxide sol studied under the 
ultramicroscope gives an intense brownish-yellow cone. 
If the same solution is diluted, the colour cone becomes 
greenish white. Likewise, a very concentrated red gold 
sol often gives a brown Tyndall cone and upon dilution a 
pure green. Concentrated Prussian blue sols show a 
violet cone and upon dilution give a_ brownish-yellow 
cone. 

The ultra-colours of colloid particles are often com- 
plimentary to their absorption colours provided very 
dilute solutions are compared with one another. Red 
gold sols usually give a green, blue or brown-yellow 
Tyndall cone. 

Polychrome ultra-images are often obtained when sols 
are examined under an ultramicroscope. A green-grey 
silver sol prepared by mixing coloured sols, has a wide 
range of particle size which gives a number of radiation 
colours. Complex reflection colours due to coarser 
micrcscopic particles may also be observed. Another 


meiealt) PROPERTIES 73 


example is given by brownish-red, commercial colloidal 
selenium. A polydisperse system contains particles of 
various sizes which give a grey colour to the solution 
due to their individual absorption colours, when viewed 
by the naked eye. Ultramicroscopic inspection of this 
sol shows the individual colours. These phenomena 
indicate a close relationship between the colour of a sol 
and its degree of dispersion. 


V 
ELECTRICAL PROPERTIES 


OST colloid particles migrate when an electric 
M current is passed through their sols (Electro- 

phoresis). This indicates that the particles 
possess electric charges. From the capacity of trans- 
porting electricity it follows at the same time that colloid 
solutions must have a conductivity of their own, apart 
from the conductivity of the dispersion medium and 
the ordinary electrolytes contained in it. Typical colloid 
particles generally possess a relatively large number of 
unit charges, 30 to 40 in contrast to the ions of ordinary 
electrolytes (acids, bases and salts). Moreover, the charge 
on colloid particles can be both positive or negative. 
Therefore, colloidal gold and ferric hydroxide may occur 
as either anions or cations. Such is not the rule in mole- 
cular disperse solutions. The changes in conductivity 
with concentration, temperature, etc., partially follow 
laws different from those applicable to molecular dis- 
perse electrolytes. Transitions of “‘ colloid electrolytes ”’ 
(McBain) to “‘ molecular electrolytes ’’ may occur as in the 
case of ferric hydroxide sols, according to W. Pauli and 
J. Matula. The term “ion’’ which has been applied to 
electrically charged particles in gases must be used, 


1 The exceptions are so-called ‘‘ amphoteric electrolytes,” as 
hydrolytic products of proteins (leucine, alanine, etc.), alkaloids, 
caffeine, theobromine, etc. 


74 


ERECTRICAL PROPERTIES 75 


therefore, in a broader sense to include electrical charged 
colloid particles. 
The methods of studying the electrical charges of colloid 
particles may be arranged in the three following groups : 
U-tube method—The simplest apparatus for the 
detection of the sign of the charge on colloid particles 
consists of an U-shaped glass tube. The tube is 15 to 
20 cm. high and has an inside diameter of 2 to 3 cm. 
Two pieces of platinum or silver wire, which are twisted 
into an horizontal spiral, serve as electrodes. These are 
stuck through two cork stoppers. Bore a hole in each 
stopper or cut’a groove along the side so 
that any gases which are formed may 
escape. Fill the tube with the colloid 
to be studied and pass a r10-volt direct 
current from the main through the solu- 
tion. If the colloid is coloured, electro- 
phoresis may be detected by a gradual 
disappearance of colour from one side of 
the tube after 15 minutes. The sign of 
the charge on the particle may also be 
deduced from this phenomenon. If the 
colloid is colourless, pass the current through it for 30 
minutes. Shut off the current, remove the stoppers 
without disturbing the liquid and pipette off the upper- 
most 10-20 c.c. of liquid in each side of the tube. De- 
termine the concentration of the colloid in each portion 
of the liquid by the methods previously given. 
Accurate studies on the rates of electrophoresis may 
be made if polarization and. electrolysis are eliminated 
at the electrodes. Polarization may be overcome if an 
U-tube is used which has a constriction in the middle of 
both arms (Fig. 8).1_ Fill the lower bulb with colloidal 
solution to the upper ends of both capillaries. Prepare 
1 U-tubes of this form have first been devised by J. Billitzer. 


Bice 3. 


76 PRACTICAL COLLOID (CHEMIST icy, 


two small plugs of filter paper and place one in each 
capillary so that the tubes are stoppered. Invert the 
U-tube and wash out the upper parts of the arms with 
distilled water. Fill the rest of the tube with distilled 
water to serve as an electrode fluid. Since distilled water 
has a low conductivity, quite a length of time may elapse 
before considerable electrophoresis has taken place. A 
very dilute electrolyte makes a suitable electrode solution. 
If possible, such an electrode should be chosen which 
exists in small amounts in the colloid to be studied. 
Use a dilute solution of KI or 
AgNO, in the experiments on 
silver iodide sols, and in the 
study of ferric hydroxide sols, 
use a dilute solution of FeCls. 
The best method is to use the 
ultrafiltrate of the colloid 
studied, for the electrode fluid. 
The ultrafiltrate may be ob- 
tained in sufficient amounts by 
simple ultrafiltration. Itis not 
necessary that the electrode 
fluid touch the stoppers when 
they are in place. Maintain 
the fluid in both arms at the same level. Carefully re- 
move the paper plugs with tweezers. If this is done 
correctly, a sharp surface boundary remains in the con- 
striction of the arm. A tube of the dimensions given 
requires a current of 80 to 110 volts. 

Use the conversion apparatus given in Fig. 9 for more 
accurate studies. 

The middle part, 3, is filled with the colloid to be studied, 
until the liquid rises slightly above the stopcocks A and B. 
Close A and B and wash the cathode and anode arms 
2 and 4. Fill the arm tubes with a conducting liquid 





ELECTRICAL PROPERTIES o 


whose concentration is the same as that of the colloid 
studied, in order to obviate diffusion potentials. Add 
the attachments 1 and 5, leaving the tap C open, 
and fill them with the same conducting liquid by 
means of a Io c.c. pipette. After removing all air 
bubbles, place the apparatus in a support and close stop- 
cock C. 

Add about 0-2 g. of CuCl, to the cathode solution and 
about 1 g. of NaCl to the anode solution so as to prevent 
polarization. Thoroughly mix the NaCl and the anode 
solution by moving the silver electrode to and fro. Use 
a plated copper foil or wire for the cathode and coat all 
but the lower end with paraffin. This precaution allows 
the current to enter the solution at points where the 
cathode is in contact with the dissolved copper salt. 
Before passing the current, open tap C, until the level 
of the fluids in both tubes is the same, then close again. 
Open A’and B and observe, every few minutes, whether 
a displacement of the middle layer occurs without the 
flow of current.. If such does not occur, the current may 
be started. ; 

Ultramicroscopic methods for investigation of 
electrophoresis—Measurements of the electrophoretic 
rate of displacement of individual particles are made by 
means of the ultramicroscope. Construct a small glass 
chamber on the object support of the microscope according 
to the process of The. Svedberg. Place two small pieces 
of glass on the right and left sides of the object support 
and two rectangular cover-glasses at the front and back. 
Seal all joints. A current of 4 to 6 volts is led into the 
chamber by two thin platinum or silver foils fastened on 
the right and left sides and connected to the conducting 
wires by pinch contacts. The chamber is finally closed 
with a cover-glass. Illumination from the front is fur- 
nished by an ultramicroscope. These experiments are 


78 PRACTICAL COLLOUD CHESS. 


described by Svedberg and Anderson in Koll. Zeitschr., 
24, 156 (1919). 

This method eliminates disturbing influences of the walls 
of the vessel. The electrophoretic rate of movement of 
the single particles may be observed and measured by 
means of a specially prepared microscopic bulb. Draw . 
out a piece of glass tube about I-5 cm. in diameter, so 
that the diameter of the constricted part is about equal 
to that of the bulb outlet. Bend the narrow part of the 
tube at right angles and cut off the larger end so that its 
upper end is level with the top of funnel attached to the 
bulb. Tightly fasten this tube to the bulb outlet with 
a short piece of rubber tubing and bind with wire 
to hold it in a perpendicular position. The bulb now 
has a funnel at both ends. Carefully introduce flexible 
o-5 mm. platinum or silver wires into the bulb through 
each of the two openings. The ends should be in the 
same plane and o-5-1-:0 cm. from each other. Use a 
current no greater than Io volts.1 Obviate overheating 
by passing the current through the sol by means of a 
pinch contact and only for short periods of time. Electro- 
phoresis is manifest by the particles, capable of Brownian 
movement, continually moving in one direction through 
the influence of the electric current. Try the experi- 
ment with a mastic sol prepared in Expt.1. Its particles 
are always charged negatively. Observe the direction of 
migration of the particles in the ultramicroscope. Measure 
the relative rates of certain particles passing over a given 
area, taking the time with a stop-watch. Other portions 
of the colloid solution may be studied by tipping the 
cuvette back and forth. This tipping will also mix the 


1 The nearer the electrodes are to each other, the smaller the 
voltages necessary. Conversely, the effect may be increased 


with a given small voltage by carefully approximating the elec- 
trodes. 


PEeciRICAL PROPERTIES 79 


products of electrolytic dissociation and render them 
harmless. 

The velocities of electrophoretic moving particles are 
calculated from the following formula : 


P 


2a eS —, 
t a 


where s is the distance in centimetres covered in ¢ seconds, 
P the potential difference in volts, d the distance in cm. 
between the electrodes, 6 a constant characteristic for 
each sol, which may be defined as the electrophoretic 
mobility of the colloid particles studied. Experimentally 





pee Se a 

Be 
Its value usually hes between I and Io x 1073 ick cee 
sec. volt 


and is approximately of the same magnitude as the 
mobility of ordinary ions. 


Capillary method for the study of electrophoresis 
—This method involves the characteristic behaviour of 
certain colloidal solutions to rise by capillarity in filter 
paper and will be described in Expt. 105. 


Galvanic couples (W. Biltz)—Make a galvanic 
couple by soldering the ends of a strip of zinc 7 cm. long 
and r cm. wide to the end of a copper strip of the same 
size and bend both strips at right angles to the soldered 
joint. Dip the ends of the galvanic element into the sol 
to be studied. The suspensoid sol flocculates in a short 
time, the positively charged particles adhere to the copper 
strip or remain in its vicinity, while the negative particles 
adhere to the zinc strip. In strongly hydrated systems, 
such as egg-white solutions, the electrophoresis requires a 
longer time, but ultimately gives the same results. 


80 PRACTICAL COLLOID CHEMISTRY 


The sign and magnitude of the electrical charges in 
colloidal solutions are extremely unstable and vary to a 
considerable extent. Investigations show that colloid 
particles bearing positive and negative charges, may 
occur in the same colloid solution. This will be demon- 
strated in the following electrophoretic experiments. 
Indefinite results are sometimes obtained due to the great 
variability of the electrical properties of the colloidal 
particles rather than to the method used. 

Expt. 100. Positive and negative colloids—Study 
the electrophoresis of 0:25 per cent. ferric hydroxide sol, 
prepared in Expts. 20-22 by the U-tube method. 
Dialysis of the sol may be performed in a parchment 
paper cup. Use parallel connections and perform the 
same experiment with mastic hydrosol (Expt. 1). A 
considerable repulsion of the ferric hydroxide colloid by 
the anode, occurs after Io to 15 minutes and the particles 
gather at the cathode. The ferric hydroxide sol is there- 
fore positively charged. Conversely, there is a repulsion 
of mastic sol by the cathode. The colloid is therefore 
negatively charged. 

Test with the galvanic couple described above, the 
type of electric charge on As,S,; and Fe(OH); sols pre- 
pared in Expts.. I2, 20-22, Examine the =precipiace 
removed from the inside of both arms of the couple with 
filter paper. The sulphide has migrated to the zinc 
strip and the hydroxide to the copper strip. 

Observe that with a short galvanic element, all posi- 
tively charged particles, ions as well as colloids, migrate 
to the nobler metal, in this case, copper. If such an 
element is used as a source of current for an electro- 
phoresis experiment in an U-tube, the motion of particles 
would occur as described in Fig. 10. Connect the 
platinum electrode with the zinc strip for a cathode and 
with the copper for an anode. The positive Fe(OH). 


Peel RICAL PROPERTIES 8I 


sol will migrate from the anode to the cathode. Colloidal 
hydroxides often possess positive charges; colloidal 
metals and sulphides, negative charges. 

Expt. lor. Changing the sign of the charge on 
colloid particles by 
varying the mode of 
preparation —Prepare 
two silver iodide or 
silver bromide sols in 
the following manner : 

molwA. = Dilute 3 cic. 
o-:IN, KI, or KBr with 
foc 0) In a 
second vessel dilute 5 
feet. OIN AgNO, Mi ts: 
Pateeonc.c. of H.0. i 
With strong agitation, 
slowly pour the KI solu- 
tion into the AgNO, 
solution, and not in the 
reverse order. 

Sol B. Dilute 4c.c. of 
o-IN AgNO, with 15 
c.c.of H.O. Inanother 
vessel, dilute 5 c.c. of 
o-IN KI with 40 c.c. of 
H.O. Pour the aque- 
ous AgNO, into the KI eS lunor 

Sol A is found to be positively, and sol B negatively 
charged when the electrophoresis is studied with an 
ultramicroscope. 

Theory states that the common ion, present in an 
excess when the colloid is formed, imparts its charge 
to the colloid particles. If the AgNO, is poured into 
the KI, the negative anion, I~, is first present in excess and 


6 





@ positive Colloid 
© negative - # 
FIG, ro. 


82 PRACTICAL COLLOID CHEMTSa aa 


it imparts its negative charge to the AgI sol. If the KI 
is poured intothe AgNO,, the positively charged cation 
Ag is first present in excess and it imparts a positive 
charge to the Agl sol. 

Expt. 102. Positive and negative ferric hydroxide 
sols—The commercial ferric hydroxide sols and those 
prepared in Expt. 21 are positively charged according 
to electrophoresis experiments, that is, U-tube method. 
Negatively charged ferric hydroxide sols may be prepared 
in the following way: Add, drop by drop, a saturated 
solution of FeCl, to 100 c.c. of 2N ammonium carbonate, 
shake continually until the precipitate first formed has 
dissolved to give a dark reddish-brown, clear fluid. If 
the solution begins to darken, wait a few minutes, so as 
not to add an excess of FeCl;. This colloid eventually 
loses its electrical charge upon dialysis. Determine the 
sign of the charge on the colloid particles in an undialysed 
sol by capillary, U-tube method, etc. 

Pour 100 c.c. of o-orN FeCl, solution into 150 c.c. of 
0-OIN NaOH solution. A bright yellow opalescent sol 
appears which seems to be less hydrolysed than the usual 
ferric hydroxide sols. It may be examined with an 
ultramicroscope. 

Expt. 103. Influence of (H*) and (OH) ions on 
the sign of the electrical charge on eg¢-white sus- 
pensoid particles (W. B. Hardy)—Prepare as in 
Expt. 92, a sol of fresh egg white coagulated by heat. 
Allow it to stand a few days so that it may better tolerate 
the addition of acid. This sol reacts neutral toward 
litmus paper, and when examined by the U-tube method, 
a weak negative charge is sometimes observed. ‘Acidify 
a small portion with acetic acid so that litmus paper 
shows a distinct acid reaction. Make another portion 
alkaline with NaOH. Perform a double experiment 
with these sols in two U-tubes or in two Michaelis 


BeECTRICAL PROPERTIES 83 


apparatus. The acidified sol shows a decided cation 
migration and hence it is positively charged. Conversely, 
the alkaline sol shows a distinct anion migration hence, 
negatively charged. These sols are suitable for an 
ultramicroscopic study of electrophoresis. 

Expt. 104. Changes in the electric charges of 
ferric hydroxide sols by filtration (T. Malarski)— 
Filter a dilute, dialysed commercial ferric hydroxide sol 
five times. Use a fresh filter paper each time and add 
to it some shreds of filter paper in order to increase its 
effect. Study the electrophoresis of unfiltered and filter 
sols simultaneously, using parallel connections. The un- 
filtered sol shows a sharply defined clear portion in the 
vicinity of the anode arm ; the filtered sol shows none or 
only a diffuse brightening. The unfiltered solution gives 
a thick dark-brown precipitate; while the filtered sol 
gives an extremely voluminous bluish-white precipitate 
in the cathode arm. 

According to T. Malarski, Koll. Zeitschr., 23, 113 (1918), 
filtration of positive sols through negatively charged 
filter paper should first decrease the amount of positive 
electricity in the sol and finally charge it negatively. 
The effect of the filter paper on the electric charge of the 
sol may be accentuated by repeated filterings. 

Expt. 105. Detection of electrically charged 
colloid particles through capillarity (F. Fichtner, 
N. Sahlbom)—Allow some coloured hydrosols to ascend 
strips of filter paper. Different properties are observed 
depending upon the charge of the sol. Metals and 
sulphide sols show a considerable separation between 
disperse phase and dispersion medium in the rising 
portion of the sol. Ferric hydroxide sols prepared in 
Expt. 21 show a distinct separation of both phases after 
a short rise. The colourless dispersion medium continues 
to ascend while the dispersed phase remains behind, 


84 PRACTICAL’ COLLOID CHEMI aii 


becomes concentrated and flocculates to form a sharp 
boundary. Fichtner and Sahlbom claim a negatively 
charged colloid will ascend the strip of filter paper un- 
separated from its dispersion medium, while the positively 
charged colloid is separated. The explanation of these 
various properties of colloids lies probably in the assump- 
tion that the filter paper wetted by the water carries a 
negative charge. While a negatively charged capillary 
adsorbs a similarly charged colloid particle unaffected, the 
oppositely charged particles are held back and collected 
so that they finally clog the capillary. 

Examine by capillarity a commercial ferric hydroxide 
sol and one preparedin Expt. 94. The former, positively 
charged, ascends only 1-2 cc. and flocculates, giving a sharp 
irregular-shaped border line; the negative sol ascends 
almost unseparated and at nearly the same rate as the 
dispersion medium. 

Examine by capillarity a 0-2 per cent. solution of 
“night blue’? and a o-2 per cent. solution of “alkali 
blue.’ Distinct differences are obtained if a concentrated 
“alkali blue’’ solution is compared with the dilute 
“night blue’’ solution. Addition of NaOH to “alkali 
blue’ and HCl to “night blue” emphasizes the differ- 
ences between the properties of the two dyes. While the 
“alkali blue” ascends readily and therefore appears 
negative, the “night blue”’ rises a short distance and 
precipitates. Therefore it appears to be positively 
charged. 

These differences may be detected more quickly by 
dropping some hydrosol on to a filter paper. Examine 
the two ferric hydroxide sols by this method. After the 
drops have spread, hold the filter paper against a light. 
A positive sol forms a broad colourless ring surrounding 
a coloured centre portion. A negative sol gives a spot 
which is uniformly coloured to the outer edge. : 


PeLeCERICAL PROPERTIES 85 


Expt. 106. Capillarity with prepared filter paper 
—Soak some filter paper with Al(OH); sol, prepared in 
Expt. 32, and allow to dry in a warm place. Study the 
capillarity of this prepared paper. The Al(OH), sol is 
no longer positively charged. Therefore, the “ night 
blue’’ solution separates upon absorption, while the 
“alkali blue’”’ flocculates at the line of contact between 
the paper and liquid. 

The sign and magnitude of the electric charge on colloid 
particles are characteristics which are more stable than 
the degree of dispersion of the colloidal suspension. 
However, simple filtration through a filter paper (Expt. 
104) may suffice to change these properties. 

The capillary method may be used with care for the 
detection of the kind of charge on colloid particles. 


VI 
EXPERIMENTS WITH GELS 


ELS are disperse systems which show both solid 
and liquid properties. As solid bodies, they 


possess a relative stability in shape and elasticity, 
especially toward rapid changes in form. However, they 
behave as fluids toward continued mechanical stress. 
They gradually assume the shape of any new container due 
to stress caused by their own weight. Diffusion of mole- 
cular disperse substances in dilute gels proceeds practic- 
ally with the same velocity as diffusion in a pure dispersion 
medium. 

Expt. 107. Mechanical properties of pastes—The 
changing liquid and solid properties of pastes may be used 
to conveniently demonstrate the properties of real gels. 
Grind 5 g. of potato starch in a mortar with 4 c.c. of water. 
Tip the mortar and allow the paste to flow in a continuous 
stream on to a glass plate. Smaller amounts give the 
drop form characteristic for liquids. Quickly rub the 
paste in a mortar with a spatula. The paste breaks into 
shell-shaped fragments showing sharp fracture surfaces. 
Therefore the paste behaves like a liquid by slowly altering 
its surface and behaves as a solid when a relatively large 
stress is applied. . 

Gels are formed as a result of change in the following 
properties of a colloid system: (1) concentration, (2) 
temperature, (3) degree of hydration usually attended by 
chemical changes, (4) formation of insoluble precipitates. 

86 


EXPERIMENTS WITH GELS 87 


Many gels having great elasticity and temporary rigidity, 
such as gelatins, may be classed as liquid-liquid systems. 
There are probably gels having the structure liquid-solid 
and solid-liquid. Silicic acid gels show emulsoid_ pro- 
perties in the first stages of formation and on ageing they 
show a suspensoid structure. This structure corresponds 
to the changes of elasticity with the age and to the first 
occurrence of crystalline Lauegrams in the aged jelly. 


ee ero LON 


Gelation includes the formation of a hydrated emulsoid, 
reversed by heating. Externally, the process of gelation 
corresponds to a large increase in the viscosity of the 
liquid sol and associated with the properties of solid 
substances such as displacement, elasticity, rigidity of 
form, etc. The process of gelation may be expressed in 
the following terms: (1) Time of gelation. Concen- 
tration and temperature being constant, the gel formation, 
similar to all colloidal changes of state, requires a certain 
time. (2) Gelation concentration at a certain time and 
temperature. Jelly formation occurs only above a cer- 
tain concentration. (3) Solidifying temperature. Time 
and concentration being constant, jelly formation occurs 
only below a certain temperature. A fourth stipulation 
may include the softening temperature of gels, which is 
usually higher than the solidifying temperature. There- 
fore, these softening temperatures do not coincide with 
the melting and freezing temperatures of crystalline sub- 
stances. 

Viscosimetry of dilute gelated solutions is a convenient 
method for studying the process of gelation. The pro- 
perties of concentrated solutions may be extrapolated 
from these results with considerable accuracy. The 
properties of a gel may be defined in terms of the condi- 


88 PRACTICAL COLLOID CHENGSit: 


tions under which it exists—concentration, temperature, 
time, etc.,—at which the colloid will no longer flow from 
the container when it is inverted. 

Expt. 108. Determination of gelation concentra- 
tion and time—Prepare in the manner described in the 
chapter preceding Expt. 66 a 12 per cent. gelatin 
solution and dilute it with warm water so as to give solu- 
tions of 12, 8, 6, 4, 3, 2, 1°5 and 1-0 penec ape 
to c.c. of each solution in a test-tube, and place a ther- 
mometer in the test-tube containing the 4 per cent. 
solution. Quickly cool all the tubes to room temperature 
or to 10° with cold water. By carefully inclining the 
tubes held in a test-tube rack, determine the time elapsed 
before the gel in each tube ceases to flow. This experi- 
ment furnishes a series of periods of gelations at various 
concentrations. Plot a time-concentration curve. The 
plotted curve may be used to interpolate the gelation con- 
centration which gives a gel at room temperature in less 
than anhour. Repeat the same experiments by warming 
the gels on a water-bath or a hot plate. Plot the corre- 
sponding time-concentration curve of gelation at constant 
temperature and by interpolation find the concentration 
of the solution which will give a gel in an hour. 

The ‘‘normal’’ gelation concentration, that is, the 
concentration at which a solution solidifies within half 
an hour, depends upon the mode of preparation. The 
concentration of the solution is approximately 2 per cent. 
at 15° C. and usually nearer I per’cent- sata ane 

Perform the same experiments with agar, soap solution 
and caproic acid. Agar is a carbohydrate and unlike 
gelatin contains no albumin. The normal gelatin con- 
centration of agar is usually less than that of gelatin. 

Expt.109. Determination of solidifying and soften- 


1 More accurate results are of course obtained by use of a 
thermostat. 


EXPERIMENTS WITH GELS 89 


ing temperatures—The temperature at which a gelling 
solution solidifies or melts depends essentially upon the 
rate of temperature change. The slower a gel is cooled, 
the higher the temperature at which it solidifies. The 
slower a gel is warmed, the lower the temperature at which 
it melts. A constant rate of temperature change, such as 
1° C. per minute, must be used in order to obtain com- 
parable results. A cooling rate of 40° to 30° C. in 10 
minutes might be called the normal rate, while cooling in 
50 minutes would be a o-2 normal 
rate of temperature change. This 
rate of temperature change, 1° C. Liquefaction 
in 5 minutes, is especially desirable 
for the following experiments : 

Use thin-walled test-tubes of equal Solidification 
size or if possible, metal test-tubes. 
Provide a water-bath (glass tank 
of 2 to 3 litres capacity) with stirrer 
and fill the tank with water at about 
40° C. Punch holes of such a size 
in a piece of stiff paper or sheet 
metal so that it rigidly holds the 
test-tubes. Place the perforated 
sheet above the water-bath. Liquefy 
the series of eight gelatin solutions used in the previous 
experiment by placing them all in the water-bath 
heated to 60° or 70° C. Leave the tubes in the bath 
and allow it to cool to 40°C. Place a thermometer 
in each test-tube and determine to within 0-1° C. when the 
temperatures of the solutions have reached that of the 
water-bath. Prepare one vessel containing ice and water 
and another containing hot water. After the contents of 
the test-tubes have reached the temperature of the water- 
bath, note the time and with constant stirring, cool the 
bath at a rate of 2° C.in 5 minutes. Regulate the cooling 


Temperature —> 





Concentration —> 


PIG. iT. 


go PRACTICAL’ COLLOID CHEMistih 


by means of cold and hot water. Determine at what 
temperature the contents of each tube solidifies by fre- 
quently inverting them. Plot the concentrations against 
the solidification temperatures. 

Determination of the liquefying temperature— - 
Allow all the tubes to stand overnight in an ice-chest, 
and the following day place them in a water-bath at 
5°C. When the contents of the tubes attain the tempera- 
ture of the water-bath, heat it at a rate of 2° C. per 5 
minutes. Determine the temperatures at which the gels 
soften in the manner described, and plot the concentra- 
tion-temperature curve. 

Comparing the solidifying and liquefying temperature- 
concentration curves, it appears that the liquefying tem- 
peratures are far above the solidifying temperatures 
(Fig. Ir). 

The following table gives an experiment personally con- 
ducted wee the author :— 

















Solidification. Liquefaction. 
| L—Ss 
es | Graphic In- 
‘Solid. Temp.|Solid. Time Lig. ~ Temp. Liq. Time | terpolation. 
=S, Minutes. | = L, Minutes. 
| 
12 tee2T 5 O |: SSkaaeee 63 OI 
8 | 19:9 8 | 2626 60 7:0 
6 | 179 18 de Sauee 55 8-3 
4 14:0(?) 37 | ae 45 9°5 
3 12°5 49 | ora 30 10°8 
2 6:9 73 [- boee 19 II‘9g 
Be aESS eee seen os Oo 13°6 
I | — —— = eae ere 
| 











The difference between solidification and liquefying 


temperatures of agar gels is considerably greater than for 
gelatin gels. 


EXPERIMENTS WITH GELS QI 


Expt. 110. Influence of preliminary thermal 
treatment on gelation—Divide a 12 per cent. gelatin 
solution into two portions. Place one portion in an ice- 
chest and the other in an Erlenmeyer flask. Fit the flask 
with a cork stopper provided with a capillary tube to 
prevent vaporization and set in a warm place.! After 
24 hours bring both solutions to a temperature of 40° C. 
The gel formed in the ice-chest quickly liquefies and the 
heated solution cools. Determine, as in Expts. 108 and 
109, the time of gelation as well as the solidifying and 
liquefying temperatures. Considerable difference exists 
between the properties of these solutions. This 
phenomenon is readily observed if a 2-3 per cent. gelatin 
solution is used. These previously treated solutions cooled 
to room temperature show a distinct difference in the time 
of gelation. The cooled portion solidifies in 1-2 hours, 
while the heated solution of the same concentration 
requires many times that amount for solidification. 

“Expt. 111. Influence of acids and alkalies on 
selation—Prepare the following gelatin solutions : 


A. .9 c.c. of 3 per cent. gelatin solution + 1c.c. H,O 


BeG.C:.c- ie fs - + 1cC.c. DC! 
. 20 
Giese. 7 Af A +1c.c. 2N HCl 
0. C.C x a + 1 c.c. N Na0H 
Io 
OQ G.c: Me rs sa + 1c.c. 2N NaOH 


Place all the solutions in a water-bath at 40° C. for 10-20 
minutes and determine the time of gelation or the gelation 
temperature as in previous experiments. 

These additions of acids and alkalies retard gelation 
measured in the manner described, yet the smaller con- 


1 The same result may be obtained by initially heating to boiling 
and packing the vessel adequately in cotton, paper, sawdust, etc., 
in accordance with the principle of the fireless cooker. 


Q2 PRACTICAL COLLOID CHEMiat 


centrations also produce an increase in viscosity accord- 
ing to Expt. 71. It is yet to be determined by a more 
detailed study of gelation whether small concentrations 
of acids and alkalies exert a gelating influence, or whether 
an extrapolation of viscosity measurements on the process 
of gelation is incorrect. 

Expt. 112. Influence of salts upon gelation— 
Prepare the following series : 10 c.c. of 6 per cent. gelatin 
solutions and 10 c.c. of normal solutions of the salts: 
potassium sulphate, citrate, oxalate, chlorate,’ chloride, 
carbonate, nitrate, bromide, cyanide, sulphocyanide, 
iodide, salicylate, etc., sulphates of potassium, sodium, 
ammonium, magnesium, calcium,? aluminium, zinc, copper, 
iron, etc. 

Maintain all tubes at 40°C. for Io to 20 minutes and 
allow to cool to room temperature. Determine the time 
required for gelation after the solutions have reached 
room temperature. These salts give the following series 
when arranged according to increasing time of solidifica- 
tion : 

The potassium salts affect the time of gelation in the 
order given above ; potassium carbonate and the salts 
preceding it in the series increase the rate of gelation 
to a greater extent than those following the carbonate 
when compared with the gelation solution as a control. 
Cyanides, sulphocyanides, iodides and salicylates of the 
concentrations given above and at higher concentrations 
practically retard gelation. These salt-effect series are 
obtained in weak acid as well as in weak alkaline solutions 
of gelatin. 

The cations with sulphate as anion markedly decrease 
the rate of gelation. There appears to be no sulphate 


1 Since KCl1O, is not so soluble, the 0-6 g. of gelatin should be 
dissolved in 20 c.c. of 0-5N KCIO, solution. 
2 Saturated CaSO, = 0:03N at 20°C. 


EXPERIMENTS WITH GELS 93 


which will retard gelation at this concentration. The 
cation effect in individual cases varies with the concen- 
tration of the salt as well as with the acid or basic reaction 
of the gelatin solution. The cation effect for weakly acid 
gelatin solutions gives the series : 


ode vie, Cu, K, NH,, Al, Fe; 
and for weakly alkaline gelatin solutions, 
eemeenescdsoNa, Me, NH,, Al, Fe, K. 


The salts used were all 0-5N concentration except for 
CaSO,, which was a saturated (0-2 per cent.) solution. 

This ionic series, representing their relation effect on 
gelatin, occurs in colloid chemistry as well as in general 
physical chemistry and is referred to as the Hofmeister 
series. 

Expt.113. Influence of non-electrolytes on gela- 
tion—Determine the time of gelation of the following 
Hyieiiitess(>..|.Levites) : 


+ I g. urea 

+ 1 g. thiourea 

+ 1 g. furfural 

+ 1 g. chloral hydrate. 
+ 1g. methyl alcohol 
+ 1 g. ethyl alcohol 

+ 1 g. propyl alcohol 
-- 1 g. isobutyl alcohol. 
C. 9 c.c. of 6 per cent. gelatin solution -++ I g. cane sugar. 

D. 9c.c. of 6 per cent. gelatin solution without addition (control). 


AYO ¢.c. of 6 per cent. gelatin tn 


B. 9 c.c. of 6 per cent. gelatin ton 


The mixtures in series A lengthen the time of gelation, 
compared with the control. In series B the higher 
alcohols likewise retard gelation in proportion to increase 
in molecular weight. Cane sugar at the concentration 
given accelerates gelation. 

For the theory of gelation, see Expts. 85 and 87. 


94 PRACTICAL COLLOID CHEMISTRY 


B. SWELLING 


Swelling involves the absorption of a liquid by a solid 
to forma gel. The process of swelling, like other changes 
in colloidal state, requires time. The rate of absorption 
of the liquid is greater at first but gradually decreases. A 
substance capable of swelling cannot absorb an unlimited 

amount of liquid—there is 
always a swelling maxi- 


A B 
mum. Swelling usually 
occurs only within a cer- 
tain temperature range, 
beyond which the absorb- 
ing substance changes 
into a colloidal solution. 
Gelatin dissolves in boil- 
| ing water without first 
forming a gel. <A small 
portion of a swelling sub- 
| stance goes into solution 
even at a lower tempera- 
ture.- The “propemys act 
swelling depends upon cer- 
tain chemical relations 
between the absorbing 


substance and the sub- 
stance absorbed, e.g. gela- 
tin swells in water and not 
in chloroform ; while rub- 
ber behaves in the reverse manner. Addition of other 
substances exert an influence on the rate and degree of 
swelling. 

Expt. 114. Qualitative demonstration of the swell - 
ing process—Cut a rectangular strip from thin, cold 
vulcanized rubber foil used in surgery. Split the rubber 


Eig. re 


EXPERIMENTS WITH GELS 95 


into two large sections (Fig. 12). Dip one portion into 
a test-tube filled with ether and repeat with chloroform, 
benzol, etc. The immersed portion shows a considerable 
enlargement even after a few minutes (Fig. 12, B). 

This experiment may be performed with a piece of 
sheet gelatin, coloured with “Congo red” or “ night 
blue.” The gelatin sheet must not be thin, for it easily 
tears when swollen. The rate of swelling is not as rapid 
as that of rubber. 

The partial swelling of a sheet of glue or gelatin is 
striking when the lower half is dipped into a dish of 
water for 24 hours. An experiment, using the thread 
method, described in Expt. 116, No. 3, is more suitable 
for the demonstration of the swelling process. 

Expt. 115. Qualitative demonstration of swelling 
in vapour—Lay a very thin, uniformly dried, coloured 
Sheet of gelatin on the table and breathe upon it. The 
leaf rolls up with a quick motion, so that the surface 
swollen by the water vapour lies on the outside and the 
dry unswollen surface on the inside. Allow the sheet to 
stand until there is no longer any difference in absorption 
on either side and the leaf will flatten. Fasten a strip of 
the same gelatin leaf on a support, allowing it to hang 
freely. Breathe upon one side. A movement in direc- 
tion of the breath first occurs, the strip remains in this 
oblique position a short time and then gradually resumes 
the original position. 

Expt. 116. Demonstration of the heat of swelling 
—Stir about 50 g. of potato meal dried at 105° C. with 50 
c.c. of water in a beaker or in a Dewar flask. A ther- 
mometer placed in the mixture registers a rise of 10° C. 
or more in a few minutes. 

*% *" * 3 *% 

The simplest method for studying accurately the 
swelling process involves the determination of the change 


96 PRACTICAL COLLOID CHEMIST 


in weight, the change in volume, or a dimension of the 
swelling substance proportionally related to the volume, 
such as its linear dimension. For determination of the 
swelling pressure, that is, the force with which the result- 
ing absorption of the fluid can be suppressed, see E. 
Posniak, Kolloidchem. Bethefte 3, 417 (1912). 

1. Weight method (F. Hofmeister); Preparation 
of the swelling plates—To measure the swelling rate 
by changes in weight, use similar shaped discs of approxi- 
mately equal weight. For studying the swelling of 
gelatin, prepare a 40 to 50 per cent. gelatin solution, as in 
the paragraph preceding Expt. 66. When cool, and still 
fluid, pour the gelatin over a glass plate, provided with 
paper strips around the edge. These strips should be 
glued to the plate with a concentrated gelatin solution 
and allowed to dry a few days before the experiment is 
done. Photographic dishes are more convenient, but 
they must have a smooth plane surface. Make the layer 
of gelatin solution 0-5-1-0 cm. thick. When using a 
photographic dish, place it horizontally on a hot plate, 
to quickly evaporate part of the water. A glass plate 
with glued paper edges may be used at a moderately high 
temperature. When the solution has dried sufficiently 
to give a stiff gel, cut the latter into one or more large 
pieces and remove it from the dish. Lay the pieces of gel 
on a clean glass plate. If plenty of material is on hand, 
round uniform pieces may be punched from the sheet of 
gelatin with a cork borer. Swelling discs may be obtained 
more economically by cutting the sheet of gelatin into 
uniform strips by means of a rule anda sharp knife. The 
strips are then cut into squares. These discs are usually 
soft and contain much water. Dry them at a higher 
temperature, but not high enough to cause softening. 
The rate of drying is determined by the appearance of 
contraction figures. Rapid drying forms twig-shaped 


EXPERIMENTS WITH GELS 97 


depressions on the edges of the disc, which may be de- 
creased in number by reducing the drying temperature. 
The drying is finished if the discs are clear and show no 
more loss of weight. The preparation of the plates lasts 
2 to 5 days, depending upon their size. Those weighing 
about 0-5 g. are suitable for most purposes and may be 
prepared within 1 to 2 days. Use swelling sheets of 
about the same initial weight for quantitative experi- 
ments, since the weight of the discs must be used in calcu- 
lating the results. After preparation of the discs it is 
best to weigh the whole material by placing the discs in 
groups whose difference in weight is 0-05 g. and keeping 
each group in a separate container. To distinguish 
between acid and alkaline reactions in the swelling discs 
a colloid dye such as Congo red may be added to the 
gelatin solution. 

Put the plates in small dishes containing the swelling 
medium, remove after a definite time, dry carefully 
with soft filter paper or with a porous plate and weigh. 
Study the rate of swelling in water vapour by weigh- 
ing. Use a pulverized swelling substance for this experi- 
ment. Place it in a weighing bottle in a desiccator 
containing a liquid of known vapour pressure. To study 
the swelling in water vapour, sulphuric acid and 
water mixtures of various concentrations are suitable. 
The partial pressure of the water vapour in the sul- 
phuric acid mixtures is given in the table on page 
98. 

2. Volume method (M. H. Fischer)—If the swelling 
substance is powdered, such as gelatin, agar, fibrin, 
starch, measure its degree of swelling by increase in 
volume on swelling in a tube of known or uniform bore. 
The swelling substance is powdered as fine as possible in 
a mortar and uniformly mixed. Calibrate the tube and 
determine the height of the powder in the tube by measur- 


7 


98 PRACTICAL’ COLLOID CHEMIST 














SPEC. GEAV 21 ou. Percent. of H SO,, pay = 
1°746 81-2 0°020 
1-659 Teh 0:048 
1°559 65°1 O-122 
1°*479 57:8 0:208 
7-426 52°7 0°306 
1°374 47-4 0°420 
1*329 42°5 0°525 
1°289 38-0 0620 
1242 32°4 0-718 
1°202 275 0°793 
i-t02 22°5 0°857 
e113 16-1 O:915 
1°052 jee 0-965 
1-000 0:0 1°000 








ing with a mm. scale. These results furnish approximate 
values of the rate of swelling. 

An Esbach tube, such as is used for quantitative egg 
albumin determinations, is suitable. A calibrated test- 
tube may also be used. 

3. Thread method—Very simple and reproducible 
swelling experiments may be performed, if the swelling 
substance is in the form of a thread. 

The enlargement of the thread cross-section may be 
negligible if the length of the thread be chosen a hundred 
times greater than the cross-section. This method is 
more rapid and more economical. The following series of 
experiments may be used to study the swelling of rubber 
threads. These threads should not be over 1 mm. thick, 
preferably thinner and cut as uniformly as possible. 
Place each thread in a thin-walled glass tube whose inner 
diameter is about double the thickness of the thread, that 
is, 2 to 3 mm. These tubes serve as a guide for the 


EXPERIMENTS WITH GELS 99 


swelling threads. Determine the original length of a 
thread by laying it onarule. Place the guide tube with 
the thread in a small test-tube or combustion tube, 12-15 
cm. long and I-o cm. diameter. Carefully pour in the 
swelling medium so that it rises in the guide tube without 
displacing the thread. Provide the large tube with a 
tightly-fitting stopper and place the apparatus in a 
horizontal position (Fig. 13). Measure the 
resulting elongation of the thread in both direc- 
tions by means of a rule or scale paper pasted 
on the tube. 

4. Osmotic method—The well-known 
osmometer may be used to measure the 
swelling of sols. This apparatus consists of 
a cell with acollodion membrane impermeable 
to colloids, and one or two tubes suspended 
perpendicularly therein. A dialysing cup, pre- 
pared in Expt. 54, may be used with an earthen 
or wire net support. Collodion membranes, 
as well as parchment cups, may be used for 
these experiments. The volume of these do 
not remain constant for any length of time. 
Parchment cups and fish bladders gradually 
expand, while unsupported collodion cups 
shrink. Supporting cells about 30 mm. dia- 
meter and 90mm. high prepared from brassor fig. 73. 
silver wire netting are very practical. The col- 
lodion membrane is prepared on the net in the following 
way: Dip the dry cell into collodion and after drying a 
short time dip into water to coagulate the collodion layer. 
Allow the water to drain and pour a collodion layer within 
and without the cell. Allow to dry for 5 or 10 minutes 
and coagulate the collodion once again. With porous 
cells the membrane may be formed directly upon the 
‘damp walls as in Expt. 54. 





100 PRACTICAL COLLOID CHESIsiim 


The cells are sealed with rubber stoppers, fitted with a 
perpendicular tube 4 mm. in diameter and which is 
approximately flush with the lower edge of the stopper 
(see Fig. 14). To obtain a tighter fit, the bore of the 
rubber stopper is swollen by moistening with benzene. 
The cell should be thickened with 
collodion at the rim of contact 
with the rubber stopper. The 
use of a second short regulating 
tube fitted with a glass stopcock 
is more convenient. The cell may 
be filled while open and the stop- 
per pressed down so as to force 
the liquid into both tubes, or the 
cell may be closed when empty 
and filled through the regulating 
tube. 

A low zero point in the higher 
tube may be obtained by careful 
suction through the regulating 
tube. Place the cell in’ay large 
beaker so that it rests on the 
bottom, in order to decrease the 
tension on the rubber cork. For 
accurate results, the beaker should 
be kept in a thermostat. 

* * 6 

Expt. 117. Velocity of swell- 
ing and swelling maximum—Perform the following 
three experiments to gain practice in the methods of 
obtaining swelling measurements : 

(a) Determine the rate of weight increase of two 
gelatin swelling discs in water by the weight method. 
The determination of the maximum swelling is difficult 
by this method, since at this point the sheets are very 





UN 





FNONNIVAQIOOULOEDOGUDUUAEATUNCATUOHN 





Fic. 14, 


EXPERIMENTS WITH GELS 101 


brittle and break easily upon drying. However, graphical 
extrapolation of the maximum swelling value is usually 
a good approximation. 

(b) Place 25 c.c. of water by means of a pipette in a 
large number of test-tubes about I-5 cm. in diameter. 
Arrange the tubes in a support to the height of the water- 
level in each tube. Use only the tubes which have 
practically equal diameters. Addo-2 or0:5 g. of powdered 
gelatin, agar, fibrin, etc., to the tubes filled with water 
by placing the powder on the surface of the water and 
causing it to sink by tapping the tube. Particles should 
not adhere to the walls of the tube. The mixture should 
be shaken at the beginning of each experiment. This 
methcd is not suitable for the determination of the rate 
of swelling, but is adequate for the determination of the 
swelling maximum. After 24 hours, measure the height 
of the swollen column by placing a perpendicular scale 
on the tube. Repeat the measurements, shaking and 
allowing to settle several times, and take the average. 

(c) Allow a thread of rubber to swell in ether, chloro- 
form, xylol, toluol according to the thread method 
described above. The velocity of swelling as well as the 
maximum swelling may be determined by using a guide 
tube of suitable diameter. The maximum swelling is 
usually attained in 1-2 hours. 

(d) Measure by the osmotic method the swelling of 
0-5 per cent. gelatin solution placed in HCl and note the 
lapse of time. A curve is obtained which is related to 
that in Expts. 117 (a) and 118. 

Expt. 118. Influence of acids and bases on the 
swelling of gelatin or fibrin—Determine by the 
weight method the swelling of gelatin plates in the 
following solutions : 

fe O-005N- HCl. <0-25N HCI. 
H.O; oor1N NaOH; 0:25N NaOH. 


102 PRACTICAL COLLOID CHOERGE ia 


Place the swelling discs in the solutions, note the time 
and determine the weights. Dry discs weighing more than 
a gram should be weighed every hour and smaller discs 
weighed regularly at shorter intervals. At first, a con- 
siderable difference is obtained for the rates of swelling 
of gelatin in acid and alkaline solutions, compared with 
the rate of swelling in water. Moreover, the swelling 
maximum does not appear to be directly related to the 
absolute increase in acid or alkali concentration. The 
effect of acid and alkali on the viscosity of dilute gelatin 
solutions (Expt. 71) is such that the viscosity varies 
directly with the capacity to swell. 

The brittleness and deliquescence of the discs in a state 
of marked swelling give accurate results for the swelling 
maximum determined by this method. Perform the 
same experiments with gelatin powder by the volume 
method. See Wo. Ostwald, Pfliigers Arch. f. Physiologie, 
108, 563 (1905) for complex absorption-concentration 
curves. 

Study the effect of o-1N acids and bases on swelling. 
The absorption maxima decrease, so that the relatively 
weak dissociated o-IN acetic acid causes a greater 
absorption than the strongly dissociated sulphuric acid. 
It is concluded, therefore, that for decreasing swelling 
maxima the anion concentration of acids is responsible 
for this effect rather than the hydrogen 1on concentration. 
Absorption experiments with fibrin by the volume 
method give the same results (M. H. Fischer). 

Expt. 119. Local acid absorption—An experiment 
on the theory of insect stings (M. H. Fischer). Pour a 
6 per cent. solution of gelatin into a crystallizing or Petri 
dish and allow to solidify. Fill a hypodermic needle or a 
glass tube having a capillary end with formic or acetic 
acid and stick it into the gel, so that a small amount of 
acid is left in it. Cover the gelatin with water. The 


EXPERIMENTS WITH GELS 103 


gelatin is strongly swollen in the places pierced even after 
I to 2 hours (Fig. 15). 

Expt. 120. Influence of salts upon the turges- 
cibility of selatin—This may be studied by using 
neutral gelatin substances just as in the previous experi- 
ment.. The action of salts on the viscosity of dilute 
gelatin solutions was studied in Expt. 71. If a 0-125 m. 
solution is used, the swelling maximum is decreased by 
sulphates, acetates, tartrates, oxalates, Muses and increased 
by nitrates, chlor- 
ides, bromides, 
iodides, etc. A 
series is obtained 
which Hofmeister 
in 1890 advanced 
on the basis of such 
swelling = experl- 
ments. By using 
various salt con- 
centrations, com- 
iex= Curves are 
obtained. The re-- 
ition of these 
curves was studied 
by Wo. Ostwald, | 
Pfliigers Arch., 111, 581 (1906). Measure the swelling 
concentration curve for NaCl with gelatin powder by the 
volume method. 

Expt. 121. Influence of mixtures of acids, alkalies 
and salts on the swelling of gelatin or fibrin (M. H. 
Fischer)—Prepare a series of acid and alkali salt mixtures 
by starting with 50 c.c. of o-1N HCland NaOH and adding 
to these 50 c.c. of normal salt solution. Observe that in 
contrast to the salt effect on approximately neutral 
gelatin, all salts considerably reduce the swelling in both 





FIG. 15. 


104 PRACTICAL COLLOID’ CHEMTS i 


acid and alkaline media. Moreover, the Hofmeister 
series applies to acid as well as alkaline solutions in which 
sulphates, acetates, etc., markedly reduce the swelling, 
while chlorides, bromides and iodides exert a lesser effect. 
The volume method is suitable for this experiment. 

Expt. 122. Influence of non-electrolytes on the 
swelling of gelatin—Study the effect of 10 per cent. 
cane sugar and 5 per cent. urea on the swelling of gelatin 
by the weight or volume method. Sugar retards while 
urea increases the swelling. 

Expt. 123. Swelling and colloid formation— 
Previous studies on the swelling of substances showed 
that in the range of maximum fluid absorption they be- 
come brittle, very soft or viscous. These characteristics 
of swelling are pronounced in alkaline solutions, in the 
higher concentrations of the alkali earth chlorides, in 
solutions of iodides, sulphocyanides, etc., and especially 
in solutions of urea. These properties are most evident 
at higher temperatures, hence it is well to conduct the 
swelling experiments at as low a temperature as possible. 
Furthermore, the absorption of a fluid by a swelling sub- 
stance and the solution of that substance by the swelling 
medium may occur simultaneously during the swelling 
of the substance. 

Determine the amount of gelatin dissolved by the 
tannin or potassium mercuric iodide method. The tannin 
test is made by adding 3 drops of freshly prepared tannin 
solution to 10 c.c. of the acidified swelling solution, which 
gives a white precipitate. The potassium mercuric iodide 
test is done by weakly acidifying 5 c.c. of the swelling 
solution with a drop of sulphuric acid and adding two drops 
of the concentrated reagent.1 In a very dilute gelatin 
solution the milky precipitate becomes visible after the 


1See Expt. 41 for the preparation of the reagent. 


EXPERIMENTS WITH GELS 105 


mercuric iodide has settled. According to H. Trunkel } 
the tannin test is positive in concentrations of I : 50,000 
and the potassium mercuric iodide test, I : 125,000. 

Increase in swelling capacity is usually followed by an 
increase in the colloidal solubility. Hence, more gelatin 
dissolves in acid or alkaline media, in solutions of iodides, 
urea, etc., than in neutral water or in sulphate solutions. 
Accurate studies have shown that no direct relations exist 
between swelling and dissolution, but that both processes 
aie independent of one another (M. H. Fischer). 
Swelling may be considered as a hydration, dissolution 
and a dispersion of hydrated colloid particles. 

Expt. 124. Swelling of rubber in various liquids 
—Determine by the thread method described above the 
maximum swelling of rubber threads in the following 
liquids: water, ethyl alcohol, acetone, amyl alcohol, 
aniline, ether, benzol, toluol, xylol, and chloroform. 
Rubber swells shghtly or practically not at all in water or 
ethyl alcohol. The swelling becomes considerable in 
amyl alcohol and increases with the molecular weight of 
the alcohol series. The dialectric constants of the rubber 
solvents decrease in the order given, with the exception 
of chloroform. Rubber swells in all liquids of small 
dialectric constants, the values of which are of the order of 
five or less. 


eo NE RESIS 


_ Syneeresis is the separation of the disperse phase of a 

gel from its dispersion medium. This process takes time 
and yields two separate phases, the solid gel intact, and 
the clear fluid above it. The best known example of this 
process is the separation of acidified milk into curds and 
whey. Uniform gels contract in the course of time with 


1H. Trunkel, Biochem. Z., 26, 462 (1910). 


106 PRACTICAL ‘COLLOID “CHE NMS Tia 


the formation of a green-yellow whey. All gels probably 
show this phenomenon. The separated phase consists 
not only of the pure dispersion medium, water, but also 
salts and small amounts of colloid. 

Expt. 125. Syneeresis of gelatin, agar, and starch 
gels—Prepare the following gels and place them in 
Erlenmeyer flasks, with 
well-fitted stoppers :— 


6, 3, 1°5,07atid are 
cent. gelatin ; 

3 .1°5 42 ieee 
per “Geli areal oc 

8, 4, and 2 pen cena, 
starch. 


The dilute solutions of 
gelatin and agar gels may 
be prepared from the con- 
centrated solutions by add- 
ing warm water and heating 
a few minutes to obtain 
uniform solutions. Prepare 
the starch paste by sifting 
the weighed amount 
through a fine sieve, with 
a brush, imtoseaeecr an 
volume of water. Shake 
continuously, make up to 
the required volume and heat 30 minutes on a water- 
bath.2. Adda few grams of thymol to the hot solution to 
prevent bacterial growth. This gel will keep when cool. 





FIG, 16, 


1 The above concentrations are approximate since it is difficult 
to obtain agar gels without a residue, which must be separated 
by filtering through cloth. 

2 Clump formation cannot be avoided by dipping in boiling 
water. 


EXPERIMENTS WITH GELS 107 


A clear fluid separates from the starch, gelatin and agar 
gels after one or two days. The greatest amount of fluid 
separates from the more dilute gels, whereas almost no 
liquid separates from the concentrated gels. Determine 
the colloid content of the fluid phase from the gelatin 
gel by the tannin or potassium mercuric iodide method 
(Expt. 123); from the agar gel with tannin and a few 
drops of HCl; and from the starch by the iodine reaction. 

Expt. 126. Synezresis of silicic acid gels—Pre- 
pare the following series of gels: 


I. 50 C.c. of Io per cent. water glass in 3 c.c. 2N HCl; alkaline. 
sO CC =, /, SS Aye G2 tC) 

Bae BOU Onn uy vA + Scan JIN HG) Sac. 

He RO Cre, * “i 690.0, 2N FICL 

SRO. 33 - Pi 2 OG aN Aba) 


Add a few drops of phenolphthalein to the water glass 
before it is mixed with the acid. The amounts of HCl 
given may be regarded as only approximate, since tests 
show commercial water glass to contain excess soda. 
First prepare the approximately neutral mixture (No. 2) 
and then make up the acid mixtures in the above pro- 
portion. All mixtures should solidify at once or within a 
few hours. 

After 24 hours or even a few days, the greatest amount 
of liquid separated from solutions I and 4 and the least 
from solution 2. Therefore, syneresis is more marked in 
weak alkaline and acid than in neutral solutions. Observe 
in the weakly acid colourless solution that the separated 
fluid has been turned pink by phenolphthalein and hence 
is weakly alkaline. Comparison between solutions 1 and 5 
shows that syneresis of silicic acid is more marked at 
high concentrations. This is contrary to the syneresis of 
gelatin, agar, etc. 

Test for the SiO, content in the decanted fluid with a 
few drops of copper ammonium hydroxide. 


1o8 PRACTICAL COLLOID CHEMiIsr 


Expt. 127. Syneeresis of a rubber gel during 
vulcanization (M. Kréger)—Add 0-6 c.c. of S.Cl., at 
room temperature, to 30 c.c. of a I per cent. solution of 
rubber (preferably Hevea crépe I) in benzine and shake 
well. The gel formation is complete after 40 minutes, 
while synzresis begins after 20 minutes. The time 
varies with the quality of rubber. 


D. PRECIPITATION REACTIONS Saks 
RELATED PHENOMENA IN GELS 


A chemical reaction which gives a precipitate in a gel 
rather than in a liquid as a medium may show a great 
variety of phenomena. Globulites and spheerolites, etc., 
sometimes occur instead of crystals with plane surfaces. 
Large well-formed crystals are produced in other cases. 
If two solutions are allowed to react by diffusing through 
a gel under definite conditions, a system of periodically 
arranged precipitate layers occurs instead of a continuous 
precipitate. Gas bubbles in gels often have a lenticular 
shape instead of a spherical one. Many experiments on 
these phenomena are only qualitative and therefore not 
easily. reproducible. The following experiments have 
been selected because they can be performed with moder- 
ate certainty. 

Expt. 128. Liesegang rings—Prepare a gel from 
4 g. of gelatin, 120 g. of water and 0-12 g. of K2Cr.0; ; 
and a solution of 8-5 g. of AgNO, in Ioo c.c. of water. 
Allow the silver nitrate solution to diffuse into the gel so 
that the insoluble silver chromate forms in periodic layers. 
This phenomenon may be illustrated by means of plate, 
tube or volume experiments. 

Plate experiment— Pour a thin layer of bichromate 
gelatin on a glass, plate or Petri dish and allow it to 


EXPERIMENTS WITH GELS 109 


solidify. Place a large drop of AgNO, in the centre of 
the gelatin sheet and cover with a dish so that the water 
will not evaporate too quickly. A series of precipitate 
rings, as in Fig. 17, form by progressive diffusion. Per- 
form the experiment on an object glass for microscopic 
study. 

















PlGe 7: 


Tube experiment—Pour into a test-tube a I5 cm. 
layer of gelatin and after solidification add a 5 cm. layer 
of AgNO, solution. The Ag,Cr,O0,, formed by diffusion 
of the AgNO;, separates in thin horizontal layers, the 
distances between which continuously increase. 

Volume experiment—Place some of the bichromate- 


110 PRACTICAL COLLOID CHEMIS ei. 


gelatin solution in an ice-chest and allow to solidify for 
24 hours. Dissolve the gel by dipping it in hot water for 
a short time or loosen it with a knife. Place the gel ina 
larger beaker which contains enough silver nitrate to 
cover it completely. The gel is lighter than the AgNO, 
solution and may be weighted down by laying a glass plate 
upon it. Allow to stand 1 or 2 days. Larger amounts 
of gel require a longer time. Then remove the gel from 
the AgNO, solution, wash the outside with cold water and 
place it ina glass dish. When cut with a sharp knife, the 
gel gives a cut surface which shows a concentric banding, 
very similar to that of an agate. 

The plate experiment is most suitable when it is to be 
preserved. The plate may be safely dried in the open, 
while the tube and volume preparations become dark 
brown, with consequent loss of the characteristic bands. 
In the experience of the author, the clearest and greatest 
number of rings are obtained at room or a lower tempera- 
ture with the above concentrations. 

Expensive silver chromate rings may be replaced by 
pretty layers of magnesium hydroxide prepared as follows : 
Swell 3 g. of gelatin, powdered or cut in small pieces, in 
300 c.c. of water and dissolve 10 g. of MgCl,.6H.O in 20 
c.c. of H,O. Pour 5 c.c. of hot water over the gelatin and 
after it is dissolved, add the MgCl, solution. A 3 per cent. 
gelatin solution is formed which is normal with respect to 
MgCl,. 

Allow to solidify in a test-tube and then add concen- 
trated ammonium hydroxide. A precipitate forms at the 
interface between the ammonium hydroxide and gelatin, 
the first isolated ring appearing ina few hours. After one 
to two days two to three rings appear which are separated 
from each other by a clear space of more than a centi- 
metre. 

Expt. 129. Forms of metallic lead precipitates in 


Pare RiIMENTS WITH GELS ty 


gels—If a zinc plate is dipped into a solution of lead 
acetate or nitrate, the lead is precipitated upon the zinc 
as a sludge or flat crystalline aggregates known as a lead 
tree. If this reaction occurs in the presence of silicic 
acid, gelatin, etc., the form of the precipitate is altered to 
a considerable extent. Prepare a series of test-tubes 
containing the following mixtures : 

I. 20 c.c. of HO + 2 c.c. saturated lead acetate solu- 
tion. 

2. 20Cc.c. of silicic acid + 2 c¢.c. of saturated lead acetate 
solution. 

The silicic acid may be prepared by mixing 10 c.c. 2N 
acetic acid with 30 c.c. of about 6 per cent. sodium silicate 
(density 1:04), which is coloured red by a drop of 
phenolphthalein. The mixture must be completely 
colourless, that is, strongly acid. A gel forms in I to 2 
hours. 

3. 20 c.c. of 2-3 per cent. gelatin solution + 2 c.c. 
of saturated lead acetate. 

4. 20 c.c. of 0-5-I per cent. agar solution + 2 c.c. of 
saturated lead acetate. 

The results obtained are as follows: (1) In the aqueous 
medium, some lead sludge and long, leaf-like crystalline 
ageregates are obtained; (2) in the silicic acid, long 
pointed needles and fir, twig-like aggregates ; (3) in the 
gelatin, short compact crystal clusters, or “‘ small trees ”’ ; 
(4) in the agar gel, beautiful ‘“‘clear’’ ramified twigs 
similar to those in silica gels. The presence of gels in- 
fluences considerably the rate and form of crystalline 
precipitations. 

Expt. 130. Forms of metal silicate precipitates— 
Fill a series of test-tubes with a 15 per cent. solution of 
sodium silicate and add to the solution a few crystals of 
the following salts: FeCl,, CaCl,, CuCl,, NiCl, or 
NiO). CoCl.-or Co(NO;)., MgCl,, -ZnCl,; etc. Shake 


112 PRACTICAL COLLOID CHEMISi. 


the crystals down to the bottom of the tube. Chlorides, 
bromides and nitrates will give better results than sul- 
phates, acetates, etc. After a short time the metal 
silicates gradually grow upward from the bottom of the 
tube. Some of the precipitates grow faster than others 
and all attain different shapes. The rapidly-forming 
figures are due to imprisoned gas bubbles rising to the 
surface. The slower-growing forms are partially due to: 
(1) formation of a precipitate which has the properties of 
a semipermeable membrane, (2) the osmotic action of 
water across this membrane, causing it to rupture and 
form anew, and (3) the difference in specific gravity of the 
lighter contents of the membrane rising in the heavier 
silicate solution. Morphologically, the type of these 
forms is tubular. 

The periodic thickenings, thinnings, and branchings 
may produce bulb-shaped ends. 

Observe that each salt under similar reaction conditions 
gives morphologically, a type of precipitate which is 
always the same. Thus, FeCl, in a 15 per cent. sodium 
silicate solution, always forms relatively thick bent tubes 
with broad end ramifications, while cobalt nitrate always 
forms long, thin, delicate shapes. The various forms of 
these metallic silicates are dependent upon the physico- 
chemical properties of the salts concerned. 

Expt. 131. Origin of native alumina—Clean a 
piece of sheet aluminium by washing with KOH and dis- . 
tilled water for a short time and then immerse it for 5 
minutes in a saturated sublimate solution diluted ten 
times. Rinse with water and dry in the air. White 
threads of aluminium oxide may be observed growing 
upon the sheet of aluminium. Microscopically, these 
threads show a structure which is strikingly similar to 
plant fibres. This alumina gel is an extraordinarily 
strong adsorption medium. 


EXPERIMENTS WITH GELS TE? 


Expt. 132. Cluster-shaped precipitate mem- 
branes—Fill a tall narrow beaker with a saturated 
solution of potassium ferrocyanide and carefully place a 
few drops of CuSO, solution upon the surface by means of 
a dropping tube. The drop, which must remain upon the 
surface of the cyanide solution, becomes surrounded with 
a perfectly transparent membrane of copper ferrocyanide. 
This membrane thickens so much in the course of an hour, 
that the copper sulphate drops appear enveloped in a 
sac, initially transparent but which finally becomes 
reddish brown and opaque. 

Perform the same experiment with a 3-6 per cent. 
solution of gelatin, which has been heated several hours 
on a boiling water-bath so that it may gel spontaneously. 
Add this solution by drops into a 5 per cent. aqueous 
tannin solution. Colourless white sacs form, which 
become dark and non-elastic after a few hours. 

These precipitate membranes are protected with a thin 
layer of gel and are the prototypes of those ‘‘ semi ’’— 
or “selective ’’—permeable membranes, which play so 
great a rdle in the theory of osmotic pressure and in general 
physiology. 

Expt. 133. Gas bubbles in gels—Add a few drops 
of ammonium carbonate solution to 20 c.c. of a Io per 
cent. water-glass solution, and pour the mixture into 
an equal volume of acetic acid. A particularly clear sol 
forms which solidifies in a few hours and which produces 
gas bubbles during solidification. 

Larger gas bubbles may be produced by heating the 
silicic acid gel a short time before its solidification. Gas 
bubbles may be formed in a 10-15 per cent. gelatin gel 
if carbonate is first added and acetic acid allowed to diffuse 
in after solidification. [E. Hatschek.]! 


1 In the experience of the author the experiment may be carried 
out with 3—6 per cent. gels to which are added, before gelling, a 


II4 PRACTICAL COLLOID CHEMIST: 


The bubbles at the time of formation or somewhat later 
may be spherical, but usually are double convex lens- 
shaped, which later become disk-shaped. 

The causes of these divergent shapes is probably due to 
the specific “‘ cleavage’”’ of the gels and to the unequal 
pressure produced by the forming bubbles. If these pre- 
parations are preserved for a long time, the lens-shaped gas 
bubbles gradually assume a spherical shape. The first 
formed lens-shaped gas bubbles are partially filled with 
syneretic liquid. Enlargement of these lens-shaped 
spaces 1S a consequence of syneeresis, the gas bubbles 
moving back and forth in the cleft space when the test- 
tube is inclined. 


E. DRYING AND FREEZING OF Gree 


Expt. 134. Figure formation in the drying of eg¢ 
white (O. Biitschi, M. H. Fischer)—Pour a few c.c. of 
fresh egg white into a small glass dish with a smooth 
plane bottom to form a layer 0-5-1 c.c. deep. Allow 
the egg white to dry in the open air or in a desiccator. 
Polygonal plates appear in the former and very fine 
spiral cracks may be observed in the latter preparation. 
Microscopic analysis with a Nicol prism shows colour 
phenomena similar to those observed with starch grains. 

Expt. 135. The drying of gelatin solutions— 
Smear a few large cover-glasses with a 6 per cent. solution 
of warm gelatin and allow them to dry in a warm place, 
such as an asbestos plate warmed by a small burner. 
Observe that the cover-glasses are bent by the contract- 
ing gel. The glass breaks with increased drying. Pour 
a small amount of the same solution upon a strong glass 
plate and put the plate in a drying oven at 100° C. After 


few drops of 2N(NH,),CO; and covered over with dilute acetic 
acid, 


EXPERIMENTS WITH GELS 115 


the gelatin is dry, dissolve it with concentrated or with 
a few drops of saturated potassium cyanide or sodium 











lente, “Lith. 


salicylate and wash the plate clean witha brush. Observe 
that the drying gelatin etched large shell-shaped figures in 
the glass. 


116 PRACTICAL COLLOID CHiMisiig: 


Solutions of animal glue are usually better than gelatin. 
The gelatin or glue must have complete contact with the - 
glass in order to obtain good results. Better contact is 
assured by roughening the glass plate with hydrofluoric 
acid. 

Expt. 136. Ice crystals in gelatin gels—Pour a 
few c.c. of hot 6-10 per cent. gelatin solution on a well- 
cleaned glass plate. Drain the excess gelatin so that a 
very thin layer remains on the plate. After the gel 
solidifies, put the plate where the temperature of the air 
is a few degrees below freezing. The formation of large 
ice crystals requires a long period at freezing temperature. 
The water, crystallizing to form the well-known ice 
flowers, compresses the gelatin in such a way that a 
gelatin ‘“‘ pseudomorphosis’’ results with “ negatives’ of 
the ice crystals. Allow the plate to come to room tem- 
perature to evaporate the water. The shapes stamped in 
the gelatin remain intact. 

H. Ambronn uses gum arabic instead of gelatin in this 
experiment. 


VII 
ADSORPTION 


DSORPTION involves the change in the con- 
centration of the disperse phase at the interface. 


Increase in concentration or positive adsorption 
is most frequent. Molecular disperse solutions, colloids 
and coarsely disperse systems can adsorb, that is, form 
more concentrated solutions at their interfaces. Adsorp- 
tion may take place at different kinds of interfaces. In 
a system consisting of a disperse phase in a liquid dis- 
persion medium, adsorption by the disperse phase may 
take place by contact with solid bodies such as charcoal, 
by contact with fluids such as shaking with chloroform, 
and by contact with gases afforded by the available 
surface of dispersoids. Adsorption is greatest when the 
interfaces in a system are greatest. Therefore, the 
specific external surface, that is, its extent divided by the 
volume or weight of the adsorbent, must be as large as 
possible. Solid adsorbents are often chosen in the form 
of powder, fluids as drops and gases as bubbles. When 
shaking a solution with the adsorbent the concentration 
changes in the solution should be uniform and rapid 
throughout the system.? 


1 After the condensation of a molecular disperse substance 
upon an interface, diffusion into the adsorbent follows. The 
latter phenomenon was referred to as ‘‘ Absorption.’ According 
to Davis, iodine is adsorbed by charcoal, but on further contact 
is absorbed, W, McBain introduced the term “ Sorption’”’ to 

117 


118 PRACTICAL COLLOID CHEMIS Tia: 


A. ADSORPTION AT THE INTE REAGi 
A LIQUID AND SOLES 


Expt. 137. Qualitative demonstration of adsorp- 
tion—Pour into a series of Erlenmeyer flasks, -50 c.c. of 
faintly coloured solutions of FeCl,, Cu(NH,).(OH)s, 
K.Cr.O,, fuchsine, crystal violet, brilliant green, methyl 
violet, etc., that is, molecular disperse systems, and also 
faintly coloured solutions of Fe(OH),, silver, gold, 
graphite, Prussian blue, Congo red, night blue, that is, 
colloidal systems. Adda gram of blood or bone charcoal 
to each of these flasks. Shake several times and filter the 
mixtures through ordinary folded filters. If the solutions 
have not been too concentrated, a Pan colourless 
filtrate is obtained in each case. 

For demonstration purposes the mixtures may be 
poured in a large filter, so as to obtain large amounts of 
colourless filtrate. Filla glass tube 3 cm. wide and 20 cm. 
long with bone charcoal and provide with an outlet. 
Filter the coloured solutions one after another through a 
column of charcoal. 

Expt. 138. Proof of the presence of adsorbed dyes 
at the interfaces—tThe disappearance of the colouring 
material upon shaking with charcoal cannot be accepted 
as final proof that the interface is responsible for the 
process. The decolorization may consist of a chemical 
decomposition of the disperse phase by the charcoal. It 
may be proven qualitatively in the following way that 


designate the whole series of phenomena in order to include 
thereby all their associated characteristics. All absorption 
phenomena are preceded by adsorption and the latter is more 
universal. Therefore, a consideration of the complex nature of 
this class of reactions has led the author from the term “ sorp- 
tion’’ to that of ‘adsorption’? in accord with the general 
practice. 


ADSORPTION 11g 


such is not always the case in adsorption as in Expt, 137. 
Shake 50 c.c. of a 0-oIN solution of “ brilliant green ’’ with 
enough charcoal (0-5-I-0 g.) to completely decolorize the 
solution and filter. Pour a portion of the charcoal from 
the residue into a test-tube containing water, another 
portion into a test-tube containing alcohol and shake each. 
The water remains colourless after the charcoal settles, 
while a green colour is imparted to the alcohol after shak- 
ing with the charcoal. This experiment shows, therefore, 
that the decolorizing of the brilhant green solution was 
not due to a chemical decomposition of the disperse phase, 
but that the dye had actually permeated the interface 
of the charcoal and liquid and could be removed again by 
a suitable solvent (Expt. 142). 

Expt. 139. Surface colours of adsorbed dyes (H. 
Freundlich)—The occurrence of surface colours is an 
indication of the extraordinarily high concentration which 
dyes can attain by adsorption on charcoal. Charcoal 
saturated with dyes in a damp state usually has a bronze 
shade. Shake roo c.c. of a I per cent. solution of crystal 
violet with a gram of charcoal and filter. The damp char- 
coal in the filter shows a brown or greenish-bronze 
shimmer similar to that of the dry pure dye. Shake some 
charcoal with water, filter, and compare the colour of the 
residue with the previous one containing adsorbed dye. 

Expt. 140. Adsorption of lead nitrate by animal 
charcoal—Shake 50 c.c. of a 0-07 per cent. solution of 
lead nitrate with a gram of animal charcoal and filter. 
The lead nitrate solution before adsorption gives a heavy 
precipitate with potassium chromate. After adsorption 
by the charcoal, the filtrate gives no precipitate with 
potassium chromate, and a slight yellow colour with H,S. 
The lead nitrate was almost completely adsorbed under 
the above experimental conditions. 

Expt. 141. Adsorption of alkaloids by aluminium 


120 PRACTICAL COLLOID: CHEM 


silicate—Naturally occurring aluminium silicates, such 
as kaolin, ordinary earth, distinct from alumina (A1,O,), 
the so-called Fuller’s earth or white kaolin, show a pro- 
nounced ability to adsorb alkaloids. The alkaloid, 
quinine sulphate, is adsorbed to the greatest extent. 

Quantitative data on the amounts of adsorbent and 
adsorbed material are difficult to obtain, since the adsorb- 
ing capacity of the aluminium silicate varies with the 
commercial modes of preparation. Fuller's earth and 
ordinary clay prove to be effective adsorbents of alkaloids 
in the experiences of the author. Shake 50 c.c. of 0-1 per 
cent. quinine bisulphate solution with ro g. of pulver- 
ized air-dried clay, filter and prove that adsorption has 
occurred by testing the filtrate with potassium mercuric 
iodide. 

Expt. 142. Influence of dilution. Reversibility of 
adsorption—Place 50 c.c. of about 0:05N acetic acid 
in each of four Erlenmeyer flasks. Add 50 c.c. of water 
to flasks r and 2, add three grams of charcoal to 2, 3 and 4 
and shake about five minutes. To flask 3 add 50 c.c. 
of water and shake again for 5 minutes. Filter the con- 
tents of 2, 3 and 4, remove with a pipette 50 c.c. from 
flask 1 and from the filtrates of 2 and 3 and 25 c.c. from 
filtrate 4. Titrate the solutions with o:rIN NaOH and 
phenolphthalein. The experiment may be illustrated from 
table on opposite page. 

The figures below were obtained from one of such 
experiments. 

The results of flasks 2 and 4 show that the amount of 
acetic acid adsorbed by 3 g. of charcoal depends only on 
the concentration of the material to be adsorbed. The 
same amount of acetic acid was present in both flasks, 
but in 2 it was dissolved in twice as much water as in 4. 
That is, the same amount of adsorbed material was present 
only in half the concentration. Observe that the con- 


ADSORPTION 20 











Number. I 2 eee! a4 
Gasepaeevic acid < 2. 4% CC.) 50 | 50 SOUR 50 
ee ees |. CC, | 50.) 50 | Oo | oOo 
ieIMOaMaeG e915) +... &g. Nh eats Vie Nie amen ee 
Additional amounts of water . c.c. O OR SO Sri 
CBs 3, os oP igi -- - + 
Pavemmirerriirale os a  .. «,C.C.| 50 50 er ee 
meacmtcimor1N NaOH. ~~. c.c. | 12°24} 5°41 | 5°45 | 4°14 
Peemceatidatsorped =. ...:. . | oO | 683 | 6-79 | 8-10 
| | | 














centration of the adsorbed substance is proportional to the 
amount adsorbed, since less of the same absolute amount 
of acetic acid was adsorbed from the dilute solution 2 
than from the concentrated solution 4. The absolute 
adsorbed amounts of acetic acid are in the proportion of 
8-1: 6-83. The comparison between 2 and 3 appears even 
more striking. The weight of acetic acid first adsorbed 
from 3 was equal to that adsorbed from 4, but so much 
water was added that the final concentration became 
equal to that of solution 2. Therefore a part of the 
adsorbed acetic acid returns into solution upon additional 
dilution, reversibility of this type of adsorption, so that 
the absolute amount of adsorption is the same (6-83 : 6-79) 
with equal end concentrations. The amount of adsorp- 
tion is determined by the final concentration of the solu- 
tion regardless of other concentration proportions pre- 
viously existing in the mixture. 

Expt. 143. Quantitative adsorption of acetic acid 
—Pour into six Erlenmeyer flasks the following amounts 
of dilute acetic acid. Prepare, approximately, the con- 
centrations given by diluting 2N acid :— 








4 PRACTICAL’ COLLOID CHEM Tai es 
Number. I 2 3 4 5 6 

C.c. of solution . 150 150 150 125 IIO 105 

Normality O°O12 | 0°025 | 0:05 3inart 0-2 O°4 























Determine by titrating with orIN NaOH and 
phenolphthalein the accurate content of acetic acid in 
each solution. For these titrations, remove with a 
pipette 5 c.c. of solution from 6, Io c.c. from 5, 25 c.c. 
from 4 and 50 c.c. from 1, 2 and 3)'s0 thatetog. cram: 
solution remain in all the flasks. 

Add to each flask, exactly 3 or 5 g. of animal charcoal 
and shake the whole series for 5 to Io minutes. Filter 
the contents of each flask separately and determine the 
amount of acetic acid in the filtrate by pipetting and 
titrating the amounts givenabove. The following person- 
ally conducted experiment describes how the results were 
arranged. 

In Fig. 19 the results are recorded graphically. The 






(3 gms.Charcoa!) 


(1gm.Charcoal) 


100 200 300 400 
Fic. 19. 


equivalent concentrations are plotted as abscisse (c—x) 
and the amounts adsorbed (x) as ordinates, on a scale 
five times as large. The adsorption curve shows that 


ADSORPTION 123 


relatively more of the disperse phase is adsorbed from a 
dilute solution than from a concentrated one and that at 
higher concentrations the amounts adsorbed approach a 
maximum called the “ adsorption maximum.”’ 

The characteristic first portion of the curve corresponds 
approximately with the adsorption formula x = k (c—x)", 
where x represents the acetic acid adsorbed by a given 
amount of charcoal, c the original concentration and 
c—x the equivalent or final concentration after adsorp- 
tion, K and n two constants. For testing this formula 





00 


10 2.0 
log(c-x) — 
Fic. 20. 


and for calculating the results use the logarithmic form of 
the equation, log x = log K +n log (c—x). This is the 
equation of a straight line which cuts the log x-axis at a 
distance log K and makes an angle with the log (c—x) 
axis whose tangent is equal to n. Kecord on a co- 
ordinate system the values of log (c—x) as abscisse and 
values of log x as ordinates, as shown in Fig. 20. If the 
equation proves to be correct the points must lie approxi- 
mately on a straight line (Fig. 20). Draw a straight line 
through as many adjacent points as possible. The values 
of the constants K and n must first be determined in order 
to use the equation numerically. Log K is equal to the 
space which the straight line cuts off on the log x-axis 


124 PRACTICAL COLLOID CHEMISTRY 


(Fig. 20). The values for the example submitted are 
0-17 and 0-72 ; the corresponding numbers for K are, there- 
fore, K, = 1-48, K,; = 5:18. The tangents of the aueies 
are equal to the quotient of the distances a : b (see figure). 
These distances are taken for maximum accuracy and 
give the values n, = 0-49 and n,; = 0-43 in the results 
plotted. Substitute the two numerical constants in the 
above logarithmic formula and calculate x. 

If Fig. 20 is plotted on a very large scale ! the calculated 
value for log x may be read directly from the plotted 
figure. Erect perpendiculars from a chosen point on the 
log (c—x) axis. The ordinate value of the point where 
the perpendicular cuts the straight line is equal to log x 
calculated. 

These simple exponential formule or SBoedecker 
“adsorption formule’ give correct quantitative values 
for only relatively dilute solutions. Experimentally and 
theoretically, the observed concentration differences 
(c—x) 1n equal weights of moderately and highly concen- 
trated solutions do not indicate the amount actually 
adsorbed. The following equation holds good for such 
cases :— 

i (c,—c) = K c™ (100—c), 

m 
where N is the weight of the solution in grams, m the 
grams adsorbed, c, the original concentration and C the 
equivalent concentration after adsorption has taken place, 
both calculated in per cent. (Wo. Ostwald and R. de 
Izaguirre). This equation gives a maximum concentra- 
tion difference or ‘‘apparent’’ amount adsorbed in 
moderately concentrated solutions and the “ apparent ” 
adsorption equals zero in a 100 per cent. solution. This 

1 The graph may be constructed advantageously on large sheets 


of co-ordinate paper pasted on cardboard and the results plotted 
with a soft pencil so that they may easily be erased. 

































































ADSORPTION I25 
Suatei= - 8. > Solution = 100 ¢.c. 
C= 
c Equili- x 
Original briumfcon-|) Amount 
concentra- centration | adsorbed 
tion in after 11¢.C. joe 
c.c.ofo-1N; adsorp- of o-1N (c—x) Kerem oh 
NaOH ition in c.c..NaOH per 
per 100 of o-1N_ |100 c.c. of Experi- Calcu- 
c.c. of | NaOH per} solution. mental. lated. 
solution. |100 c.c. of 
filtrate. 
12:0 13 4:7 | 0863 | 0°672 4°7 4°5 
26:0 18-9 yet E277.) 0-851 he 7*I 
52°3 42°3 10-0 1'626 I*000 10-0 10°5 
105°2 90°5 14°7 T°957 | 1167 | 14°7 15°3 
220°8 198-2 22:6 Z°207 1°354 22:6 224 
444°8 AII‘7 33°1 2°O15 I°520 eta 32°1 
/ 
foots, nh = 0°40. 
SeareOd ==. 2, ;) Solution = 100 C.c. 
L272 3°6 8-6 0°556 | 0°934 8:6 9:0 
24°4 10°8 13°6 1°033 1°134 13°6 14°4 
49°4 27°8 224 1°431 I°350 2254 21°4 
100°4 66:8 33°06 1°825 1°526 33°06 BEtO 
198°5 I51°5 47°0 2:180 12672 47°O 45°0 
385°8 320°6 59°2 2°514 1+772 59:2 62:6 
Fee STS. n=O Ag. 


equation represents cases which seldom occur in practice 
and assumes that only the disperse phase and not the 
dispersion medium is adsorbed. Nevertheless, such is the 
general case. A discussion of this equation is given by 
Wo. Ostwald and R. de Izaguirre in Koll. Zeitschr., 30, 279 
(1922). 

Expt. 144. Adsorption of Crystal ponceau and 
methylene blue by wool; L. Pelet-Jolivet (electro- 


126 PRACTICAL’ COLLOID” CHEMI iia 


chemical adsorption)—Use a 0-2 per cent. aqueous solu- 
tion of crystal ponceau and a 0-05 per cent. solution of 
methylene blue to prepare the following mixtures : 


Crystal ponceau: 1. ro c.c. of solution. 


DE ONGEC te », -+10drops2N HCl. 
Ast LO CzOver, » +10 drops 2N 
NaOH. 
Methylene blue: 4. Io c.c. of solution. 
5. IOCc. ,,° 4,  — igharops Netier 
6. IOC.C. ,,. 4,5) == LOU eee 


Place a few white wool fibres in these mixtures, leave 
them 20 to 30 minutes at room temperature and then wash 
thoroughly with cold water. Observe that the wool 
fibres are not affected by the basic or neutral solutions but 
are distinctly coloured by the acidified dye, crystal 
ponceau. The reverse occurs in the methylene blue 
solution. There is an indistinct colour adsorbed from the 
fibre by the acid dye, a distinct colour with the neutral 
dye and a very intense colour with the alkaline solution. 

These results may be explained by electrical or electro- 
chemical considerations. The wool is charged negatively 
in the presence of an excess of (OH) ions and positively 
in excess of (H*) ions, This is the caseuwithma is 
relatively indifferent substances (J. Perrin). Crystal 
ponceau is an acid and methylene blue a basic dye. If 
the wool carries a definite positive charge in acid medium, 
the oppositely charged anion of the acid dye, crystal 
ponceau, isadsorbed. Conversely, in an alkaline medium, 
the wool is negatively charged and the positively charged 
cation of the basic dye, methylene blue, is adsorbed. 
Wool is a weak acid and exists in water in the form of a 
massive negatively charged anion in association with the 
hydrogen ion. In a neutral medium, the wool should 
show an alkaline reaction toward both dyes ; the experi- 


ADSORPTION 127 


ment shows this to be the case.!' In contrast with the 
adsorption of acetic acid by charcoal, this electro-chemical 
adsorption is irreversible (L. Michaelis). 

Use filter paper strips instead of wool. Choose a 
shorter time of reaction or a more dilute solution, else 
the difference in the reaction is insignificant. 

Expt. 145. Specific dye adsorption by silicic acid 
and aluminium hydroxide gels—The pure commercial 
preparation “‘ asmosil’’ or Patrick’s “ silica gel’’ may be 
used as the SiO, gel, while the commercial native alumina 
is a suitable Al(OH), gel. These gels may be prepared 
by pouring water glass into concentrated HCl and 
aluminium chloride into ammonium hydroxide. Thor- 
oughly wash the two gels. Fill two test-tubes or flasks 
with a few grams of SiO, gel, two others with Al(OH); 
gel. Test the properties of the gels with a 0-01 per cent. 
solution of methylene blue and a corresponding solution 
of “‘ patent blue.”’ Allow the mixtures to stand a while 
and decant or wash into a filter. The silicic acid gel has 
irreversibly adsorbed the “‘ methylene blue ’’ and has not 
at all or only faintly adsorbed the “ patent blue.” Con- 
versely, the aluminium hydroxide gel has_ strongly 
adsorbed the “ patent blue,” but has only faintly adsorbed 
the methylene blue. 


B. ADSORPTION AT THE INTERFACE OF 
TWO LIQUIDS 


Expt.146. Adsorption of colloidal copper sulphide 
at the interface, water-chloroform (W. Biltz)—Pre- 
pare two copper sulphide hydrosols as follows : 

A. Add 1-2 c.c. of a dilute solution of copper sulphate 

1 Further data is given by L. Pelet-Jolivet and co-workers in 


Koll. Z., 2, 225 (1905), which contains a complete bibliography, 
or in The Theory of Dyeing, by the same author, Dresden, IgTto. 


128 PRACTICAL COLLOID CHEMISTRY 


or copper chloride to a mixture of 90 c.c. H,O + Io c.c. 
of freshly saturated hydrogen sulphide water, until a 
weakly turbid dark-brown sulphide sol is attained. 

B. Toasimilar mixture of water and hydrogen sulphide, 
add drop by drop, 1-2 c.c. of a dilute solution of copper 
ammonium hydroxide. Prepare the latter by mixing the 
CuSO, solution used in A with NH,OH until the resulting 
precipitate again dissolves to give a clear solution. A 
dark-brown sol is obtained, but it is clear in contrast to 
the sol prepared in A. 

Shake 15 c.c. of both sols with 2-3 c.c. of chloroform. 
Sol A is completely decolorized after shaking for a few 
seconds. The copper sulphide is adsorbed at the inter- 
face, water-chloroform, and sinks with the chloroform. Sol 
B is only slightly decolorized after much longer shaking. 

The reaction of the sols is the cause of this difference in 
behaviour. Sol A is acid and sol B is alkaline as a result 
of their modes of preparation. If sol B is weakly acidified 
~so that it is not flocculated within 1 to 2 hours, it may be 
adsorbed like sol A. 

To obtain the sol in the upper layer of the mixture, 
the experiment should be performed with benzol. 

Expt. 147. Adsorption of gelatin at the interface, 
water -benzol—Prepare a dilute gelatin solution which 
gives a distinct precipitate with the tannin test—IOo c.c. 
of solution + 1 c.c. of 10 per cent. tannin solution + I 
c.c. dilute H,SO,. <A o-oor per cent. gelatin solution is 
suitable. Shake 15 c.c. of this solution with 5 c.c. of 
benzol or xylol for at least 5 minutes and filter the white 
emulsion through a moistened filter paper.! Perform 
a tannin test on the clear filtrate in exactly the same way 
as before, shaking the solution with benzol. The tannin 
reaction, if it has not disappeared, has become consider- 


1If the filter paper is not previously wetted the pores are 
clogged by the benzol, with consequent slowing of the filtration. 


ADSORPTION 129 


ably weaker than before adsorption. Therefore adsorp- 
tion of the gelatin has taken place at the surface of the 
benzol drops. 

Expt. 148. Adsorption of a coarsely disperse 
powder, such as charcoal at the interface, liquid- 
liquid—Prepare a I per cent. aqueous suspension of 
animal charcoal, by previously grinding it as uniformly as 
possible. The suspension settles slowly. Shake about 
15 c.c. of the suspension with benzene, xylol, chloroform, 
carbon tetrachloride, etc. Analmost quantitative adsorp- 
tion of charcoal particles occurs at the interface between 
water and the liquids named. 

Expt. 149. Separation of coarsely disperse mix- 
tures by selective adsorption at the interface, liquid - 
liquid (Flotation)—Prepare a finely-ground mixture of 
9 parts of animal charcoal and 1 part of aluminium silicate 
such as clay. Shake 2-3 g. of this mixture with 100 c.c. 
of water to form a suspension and decant once or twice 
from the coarse particles of clay which may be present. 
The suspension should appear black or greyish black. 
Shake this suspension with one of the liquids mentioned 
in the previous experiment, such as benzol. The char- 
coal, not the clay, is adsorbed at the interface, benzol- 
water. The mixture separates into two sharply-defined 
layers when poured into a test-tube. The upper benzol 
layer is coloured black by the charcoal adhering all 
around it and not by internal absorption. The lower 
aqueous layer is coloured greyish white by the clay which 
remains unadsorbed. A sharp separation of the mixture 
is thus produced. 

This experiment demonstrates the typical phenomenon 
of a series of technical processes known as “‘ flotation ”’ 
and have attained great importance and development in 
recent years. This process consists in freeing and purify- 
ing graphite from the earthy gangue by shaking with a 

7 


130 PRACTICAL COLLOID CHEMIST 


suitable hydrocarbon. Sulphide ores are also concen- 
trated at such lquid-liquid interfaces, but other sub- 
stances are concentrated at the liquid-gas interfaces, while 
gangue-like substances remain behind in the aqueous 
dispersion medium. The application of this process is due 
to its ability to accomplish extensive disintegration of 
material. Sludges are too difficult to handle by the usual 
smelting process and may be recovered by this method. 

The following simple experiment demonstrates the 
essentials of many flotation processes : 

Expt. 150. Flotation of printed and unprinted 
pieces of paper—Cut a few pieces from a printed, sized 
paper ; a few from a sheet unprinted on either side, anda 
few from a heavily-printed page. Shake these pieces in 
water contained in an Erlenmeyer flask until they sink 
to the bottom. Cover the water with a thin layer of 
light mineral oil, the dispersion medium of black print, 
and shake vigorously. After separation of the layers, 
observe that the printed pieces remain in the upper layer, 
with the black printed side upward, while the unprinted 
pieces of paper remain at the bottom of the flask. 


C. ADSORPTION AT THE DIOU ars 
INTERFACE 


Expt. 151. Peptone membranes—Weigh 1 g. of 
peptone and dissolve it in 200 c.c. of distilled water to 
give a 0-05 per cent. solution. Put about half the solution 
in a crystallizing dish about 15 cm. in diameter. Dilute 
a portion of the remainder ten times (0-005 per cent.) and 
fill a crystallizing dish of similar size with this solution. 
Float a new, thin sewing needle upon the surface of the 
liquid in each dish by holding the needle in the middle 
with dry finger-tips, immersing them gently into the 
liquid and carefully releasing the needle. Bring a 


ADSORPTION 131 


horseshoe magnet near the needle without touching it. 
Withdraw the magnet rapidly so as give the needle a rota- 
tory motion. The needle oscillates through an arc of a few 
degrees and then comes to rest. The needle itself is not 
magnetic and automatically assumes a polar orientation. 

Allow the needle to float undisturbed upon the peptone 
solution for 20 to 30 minutes and repeat the magnet 
experiment. Observe that the needle still rotates when 
under the influence of the magnet. If this magnetic 
influence is not too strong, the needle will not come to 
rest in a new position, but will again return to the original 
position. This backward rotation is not due to polar 
attraction but to the peptone accumulating in a higher 
concentration at the interface of the peptone solution 
and water vapour. This interface assumed the proper- 
ties of an elastic membrane, that is, a thin gel membrane 
which is not discernible by the naked eye. 

This membrane forms more rapidly in more concen- 
trated solutions. However, it may be formed in a very 
dilute solution after about 24hours. At the end of this 
period, the membrane of a 0:05 per cent. solution is so 
strong that the needle will only move when the magnet: 
is very close to it. Its motion is usually longitudinal, 
not lateral or rotatory. Tear the membrane around the 
needle with a glass rod. The needle now behaves toward 
the magnet as in the beginning of the experiment. 

Another modification of the experiment is to use a 
sewing needle magnetized by stroking. Such a needle 
maintains polar orientation on the surface of a fresh 
solution during rotation of the dish. After the solution 
stands 20-30 minutes, the polar adjustment becomes 
slower and rather indefinite. After 24 hours, the mem- 
brane is usually so rigid that the dish may be turned in 
any desired position without the needle being able to 
resume polar orientation. 


VIII 


COAGULATION, PEPTIZATION AND 
RELATED PHENOMENA 


OAGULATION is the decrease in the degree of 
e dispersion of colloidal solutions by formation of 

microscopic and larger particles. This process 
results in a settling of the coarsely disperse particles with 
a simultaneous decrease in the volume of the dispersion 
medium. The particles in a stable colloidal solution 
remain dispersed because of a definite hydration or 
solvation with the dispersion medium. The process of 
coagulation involves a decrease in this hydration and 
hence a mutual separation of the two phases. Un- 
flocculated colloids are termed sols, while coarsely disperse ~ 
colloids are known as gels. There are a number of 
methods for the flocculation of a colloidal system. The 
phenomena studied in the previous experiments involved 
flocculation by the addition of electrolytes. 

The size of the particles which determine the state of 
coagulation cannot be defined because of the great number 
of transitions possible between colloid and coarsely dis- 
perse particles. Flocculation, like all colloid changes of 
state, require time, but the rate of flocculation may vary 
considerably, depending upon the experimental conditions. 
Therefore, flocculation can never be defined like gelation 
in terms of such values as flocculation point, concen- 
tration, time or temperature. However, the course of 
flocculation may be followed by plotting curves of the 

132 


COAGULATION AND PEPTIZATION 133 


rate of change in the size and degree of solvation of the 
coalescing particles. A direct accurate method consists 
of ultramicroscopic counting of the number of particles 
ina given volume of sol dur- 

ing flocculation. This method 

is applicable only to sols stopcock 
completely resolvable ultra- 
microscopically and is only 

practical in certain cases. 
miictewmare, Other direct 

methods of more general ap- 

plication. Another simple 
principle, of use in these floc- 

culation experiments, is_ to 

follow the rate of sedimenta- Comparison 
tion. If the right and left Wie 
arms of a U-tube are filled 

with equal volumes of two 

liquids having different den- 

sities, the heights of the 

liquid columns vary inversely 

as their densities. If the 

density of one of the liquids 

changes, then the difference 

in level will change until the 

densities of both liquids be- 

come equal. Sucha density 

change occurs ina coalescing 

solution if the upper liquid 

decreases in the concentration Hae 

of the disperse phase. The 

two-armed flocculation apparatus involves this principle 
(Fig. 21).1 It consists of a U-tube 130 cm. long, as shown 









UR TED 
“MN 
a 
a 
La) 





Sol 
Tube 


‘ Outlet with 
E glass stopper 


1 Wo. Ostwald and F. V. v. Hahn, ‘‘ On a Kinetic Flocculation 
Meter,” Koll. Z:, 30, 62 (1922). 


134 PRACTICAL COLLOID CHESTS Tis 


in the figure. The narrow arm, 3-7 mm. wide, is called a 
comparison tube and is provided with a stopcock above and 
a bulb blown at the lower end. Beyond the bulb there is 
a constriction leading to the wider tube of 6 mm. diameter. 
The constriction prevents mixing of the comparison 
liquid with the sol to be studied. The sol tube has a 
bulb just below the opening of the comparison tube. 
The lower end of the sol tube consists of a pear-shaped 
appendage, 8 cm. long and 8-10 mm. wide, terminating in 
an outlet closed with a ground-glass stopper. The 
upper end of the sol tube is somewhat widened to obviate 
bubble formation on filling. The readings are made by 
means of a scale, 130 cm. high extending a mm. below 
the bulb of the comparison tube. 

The procedure is as follows: First, add the comparing 
liquid, such as distilled water, to the apparatus until it 
reaches the 120 cm. mark in both arms. Close the 
stopcock in the smaller arm and open the glass stopper 
at the bottom of the large tube. The sol tube is emptied 
and the comparing liquid remains in the smaller tube 
because of the constriction. Rinse the tube with the 
sol and fill it up to the 115 cm. mark. Then open the 
stopcock and observe the displacement of the two liquids. 
Read the difference in height at definite time intervals, 
depending upon the rate of flocculation and the accuracy 
desired. These differences may be calculated from the 
readings on the comparison tube. The single arm 
flocculation tube is more sensitive, especially against 
temperature variations. 

The following coagulation measurements on a mercury 
sulphide sol in the double arm flocculation tube are 
illustrative. 

There are a number of simpler indirect methods for 
determining the rate of flocculation. These methods 
depend upon gradual changes of a physico-chemical 


COAGULATION AND PEPTIZATION 135 











Time after addi- Height in sol Height in com- 
tion of coagulant tube. parison tube. Difference. 
Minutes. mm. mm. 
2 272-4 1g Wha 39°8 
4 £272°4 IAi2"2 39°8 
6 1272°4 1g12°2 39°8 
Zz 12724 312-2 39°8 
8 27274 31272 39°8 
9 ‘275 * T310°4 Bes 
Io 1273°6 1309°6 36:0 
a2 5274-1 1308-7 34°6 
15 1275°1 1306°9 31°8 
18 2295757 13058 30°1 
22 1276°8 1303°8 27°0 
26 Re 7A 1302°6 25°2 
30 £27'7°0 1302°5 24°60 














property of the sol during flocculation, 1.e. turbidity, 
surface tension, viscosity, etc. Examples of such changes 
during coagulation are given below. 

The kinetic methods involving a continuous change in 
one of the properties during coagulation have as yet 
not been sufficiently developed. As a result, the deter- 
mination of coagulation points is more prevalent. In- 
direct results may be obtained as follows: Determine (1) 
the concentration of an added substance which will 
produce a marked change in the turbidity of a sol in a 
given time, i.e. within 1, 2 or 24 hours; (2) the concen- 
tration of an added substance at which a microscopic 
‘flocculation is produced ; (3) the concentration at which 
the disperse phase is held back by filter paper with pores 
of uniform size; (4) the concentration at which the 
flocculating system attains either a colour shade or 
turbidity of a prepared comparison solution, etc. 


136 PRACLICAL COLLOID CHEMIsSii 


A, FLOCCULATION OF SUSPERSGr 


Electrolyte coagulation of typical suspensoids is 
characterized by the small absolute concentration of 
electrolyte necessary for precipitation. This frequently 
amounts to only a few millimols of electrolyte per litre 
of sol. The valence of the coagulating electrolyte is of 
great importance. In general, a colloid is precipitated 
by a much smaller concentration of polyvalent than 
monovalent ions. Furthermore, the sign of the charge 
on the colloid is also significant. In the case of positively 
charged colloids, the valence and the nature of the nega- 
tively charged anions influence flocculation. Conversely, 
the cations have a similar influence on negatively charged 
colloids. The absolute concentrations of the electrolytes 
precipitating a colloid depend upon the conditions of 
preparation of the sol, the degree of dispersion, the 
concentration, etc. Since concentration varies with 
temperature, determinations made on the same sols 
under defined conditions would be comparable. 

Expt. 152. Qualitative demonstration of the elec- 
trolytic flocculation of suspensoids—Fill two similar 
large cylindrical or preferably parallel walled vessels 
either with mastic sol prepared in Expt. I or with dilute 
black India ink. Pour a few c.c. of concentrated alum 
solution into one of the vessels, stir and allow to stand 
undisturbed. Large white curds appear in the vessel 
containing the electrolyte within 20-30 minutes. More 
rapid flocculation may be produced either by using a ~ 
clear As.S, sol, prepared in Expt. 12, or a semi-transparent 
solution of commercial collargol. A large addition of 
alum causes a distinct turbidity immediately or within 
a few minutes. Examine the sol microscopically and 
observe that the identity of the sol is gradually obscured 


COAGUEATION AND PEPTIZATION 137 


by addition of electrolyte, whereas upon lateral inspection, 
the sol radiates light more intensely. 

Flocculation may be demonstrated qualitatively by 
using the sensitive red gold sol, which changes colour at 
once upon addition of a few c.c. of HCl or alum. A 
0-01 per cent. solution of Congo rubin may also be used 
to demonstrate the sudden change upon addition of almost 
any electrolyte with a subsequent coarser flocculation 
within a few hours. 

Expt. 153. Electrolytic flocculation of negative 
arsenic trisulphide sols—Prepare about 50 c.c. of a 
Ge teeeeowesoledssin Expt. 12. Pipette ro c.c. of the 
sol into a series of well-cleaned test-tubes having prac- 
tically the same diameter. Into another series of test- 
tubes place by means of a I0 c.c. graduated hand burette 
various electrolyte solutions, such as HCl, KCl, CaCl,, 
AICl,, K.5O0,, potassium citrate, diluted with water so 
that the total volume of each solution will be just Io c.c. 
Pour each salt solution into a tube containing the As.S, 
sol and thoroughly mix by pouring the solutions back and 
forth a few times. Thus, 20 c.c. of the reaction mixture 
half sol and half electrolyte is obtained. The following 
series of mixtures serve as an example. 

As.5,; obtained by passing H.S into 500 c,c. of a 0°5 
per cent. AsO, solution for 5 minutes. 


(aedaCl. 
2°5c.c. o IN HC]+7°5 c.c. H,O-+10 c.c. As,S, sol=12-5 millimols 
Beers eave 1 15°0 r» v =25 r 
PoceciN  ,, -+9:0 me ee —=50 on 
270 C.Cs- ; » +80 a re ==100 ve 

ee Be 


5-0 c.c. o-IM KCl-++5:0 c.c. H,O-+10 c.c. As.S3 sol = 25 millimols 


0°5 c.c. 2°0M KCl-+-9°5 4 50 on 
OCs. ang 9 +9:0 ” ” = 100 oe) 
ZO. €.C; » +8:0 ==, 200" 82, 


a) 2) +) 


138 PRACTICAL: COLLOID CHEMS is 


(c) CaCl,. 
0°5 c.c. o-oI1M CaCl,-+-9°5 c.c. H,O+10c.c. As,5S, sol=o-25 milli- 
mols 
I°OC.C. %, » +9:0 * os =0-50 __,, 
2°0C.C. a » +8-0 5 a == 1-009 
470 ©.C, i. » +6-0 & - == 2°OOrciaes 
(2). AICI,. 
0:5 c.c. 0:00IM AICI,-++-9°5 c.c. H,O+10¢.c. As,S,; sol=0-025 milli- 
mols 
TOC, 02) aes » +9:0 :, re ==0:-0507, 
POuG t: i » +8:0 a ue = 0-10iga-- 
A*ORO,C, aaeee, » +6:0 Ee " =0°20 af 


(2) Se RaSOn 
0:5 c.c.1'-ONK,SO,-+-9°5 c.c. HzO +10c.c. As,5,sol=: 25 milli- 


_ mols 
I°o C.C. a) +9:0 >) 2) —— 50 >”? 
2:0 €.C. ae +8:-0 ss ye =—100 f 
AsO C1 es -+-6:0 * is ==200 


(f) % K, citrate. 
0:6c.c.2°0N K, citrate+-9°4 c.c.H,O-+10c.c.As,S, sol=62+5 milli- 


mols 
1°25 C.c, ;, +8-75 _ * == 1255 
Pao bs - +7+5 i) Fs ca Cees 
50 C.C. ¥ +5:0 fi A seo SOO ere 


The concentration of the electrolyte which causes a 
distinct turbidity immediately or after one hour may 
be regarded as the flocculation value. Thus, in the 
previous experiment, the sol did not immediately become 
turbid with KCl at 25 millimolar concentration, but 
distinctly so with a concentration of 50 millimols. The 
flocculation value les between the two concentrations 
and may be accurately determined by the use of inter- 
mediate concentrations. 

The figures in heavy type show that in the above 
example, the flocculation values of the three chlorides 
are approximately as 80:0-8:0-08 or as 1000: 10: TI. 
The trivalent Al*** ion is by far the strongest electrolyte, 
since the least concentration is required to flocculate the 


COAGULATION AND PEPTIZATION 139 


sol. K.SO, and potassium citrate behave like KCl and 
their flocculation values are approximately of the same 
magnitude. -HCl is not really a strong flocculant and 
remains far below the flocculation values for CaCl, and 
AICl;. These results indicate that the cation or positively 
charged ions are responsible for the flocculation values 
for a negative arsenic trisulphide sol and that the activity 
of the cations greatly increases with their valence. The 
following tables show the flocculation values of other 
electrolytes. It indicates also that the valence of the 
cation does not play the only role. Determine, for 
example, the flocculation value of the univalent morphine 
chloride (mol. wt. 321°5). 

Other sulphide sols, negatively charged metal sols and 
mastic sols, behave like the As.5; sols. 


HILOGCCULATION OF AS,5, SOLS. 








According to 
ite Le 
and H. Picton. 


According to 


According to 
H. Schulze. 


Electrolyte. H. Freundlich. 














Univalent cations, c = millimols per litre. 











CAcciG acidu. .. ~ | “Ca.-14900) | — —- 
Prete ee) ss | Ca. 1290) — — 
(eoraliteachl | 4.) ‘Ca, 373) — — 
etipeti eee es | Ca. 275) — —- 
eelomeltrate., .°.. = _ > 240 
Meacetatee ihn i.) S| — — IIO 
PEDO en ss | — 124°4 — 
TG Ns oe a —- 109°0 — 
[ELON ar A i 185°4 — 58-4 
4+ K,Fe(CN), eee | 181-2 — = 
Sodium acetate > . | 154°3 — — 
LNG SCs Saas 1510 123° 65°60 
% pot. oxalate . . | 13172 ~ — 
COE = LT7°6 1047 50:0 
Ss) 0 aia 1090 137°4 — 
LGR) Sie 107°3 102°2 — 
eee we a, 5 | — 1 7°O — 





* Kgtartrate. . . | 104°3 —- — 


140 PRACTICAL COLLOID: CHEMIS ties 
FLOCCULATION OF AS,S3 SOLS (continued). 





Electrolyte. 





According to 
H. Schulze. 


According to 
S. E. Linder 
and H. Picton. 


According to 
H. Freundlich. 





4 K,Fe(CN), 

NaNO, 

KCl 

KO. 

NH INOS. 

NH,I 

NH,Br 

NETGG! 

Pers 

NaBr. 

Na Cl ena et 

} (NH,),SO,. 

4+ 11,50; . 

HNO, 

EtG] 

HI 

HBr : 

Guanidine nitrate 

1135074 ; 

Strychnine nitrate . 

Aniline chloride. 

p.-Chloraniline chlor- 
1G ae) a 

Morphine chloride 

New fuchsine 


Bivalent cations, c = millimols per litre. 


MgsoO, : 
[Pe(NH,4) (SO4)» 
MnsSO, ; 
Bes; 

Cos0, 

ZnO, 

NiSO, 

CaSO, 

Nil. 

CdCl, 

Feces 

Co(NO3)» 








100°5 
100°4 
979 


277 


205 


Bao 
3°03] 
2% 
zy ty | 
1°86 
1°88 
2°64 














T1o0°8 
[97-9] 
73°9 
73°9 
73°9 
62:9 
IOI‘O 
I09:0 
103°5 
95°8 
92°4 
ues 
58°7 
5755 
56-0 
I:60 


—- 


2°10 
2°02 
2°02 
I-96 
1°68 
1°65 
I-60 
[752 
1-46 
1°42 
1°37 





30°1 


30°8 


16°4 


8-0 
2:52 


T°08 


0°425 
O-114 


o-8Io 


COAGULATION 


AND PEPTIZATION I4I 


FLOCCULATION OF As,S; SOLS (continued). 


Electrolyte. 





According to 
H. Schulze. 





Zils 
CaCl, : 
Ca(HCO.,) >» 
Gabre 
MgBr, 
CoCl, 
Sr(NO,), 
Ca(NO,), 
pit Gans 
Cu(NOs) > 
BaCl, 
MgCl, 
Ba(NOs)¢ 
CdCl, 
UO,(NO3). 
CdBr, 
CdSO, 
CuSO, 


Al(NO,), 
NH ,Fe(SO,), 
KCr(SO,)» 
KAI(SO,) » 
KFe(SO,)» 
NH,Al(SO,), 





According to 
Sp. Linder 


| and H. Picton. 


According to 
H. Freundlich. 





Trivalent ca 


0-310 
0-123 


1°34 
feo 
+31 
“31 
-20 
20 
29 
23 
23 
18 
‘T4 
‘T4 
“OI 


Le ee ce Pe ee | 


0°954 
0°924 
O:oII 
0:899 
C322 
0°225 
tions. 
0:216 
O°154 
0°136 
0:080 


0°074 
0°074 
0°074 
0:062 
O:102 
0:092 
0°040 





0:040 











0:685 
0-649 


142 PRACTICAL. COLLOID *CH Riis 


Expt. 154. Electrolytic flocculation of copper 
sulphide hydrosol—Prepare a copper sulphide sol 
(Expt. 146) by dropping dilute copper ammonium 
hydroxide into dilute H.S water and pour 20 c.c. of the 
mixture into an Erlenmeyer flask. A drop of this sol 
is placed upon a filter paper by means of a glass rod. 
If a thin uniform light brown spot forms, the sol is negative 
(Expt. 105). If the sol does not spread uniformly but 
forms a “mirror” or a small spot with sharp edges 
surrounded by a larger circle, it is too coarsely disperse 
and the experiment does not apply. Add from a burette 
containing normal KCl or MgCl,, successive, small 
definite amounts of electrolyte, shake, and after each 
addition, test on filter paper. A concentration is soon 
attained at which the sol no more spreads uniformly 
over the paper, but forms a “ mirror’’ with sharp edges 
and is surrounded by a ring of the colourless dispersion 
medium. This concentration of electrolyte added may 
be accepted as the precipitation value. 

If a series of mixtures are prepared simultaneously as 
. in Expt. 153, the precipitation value may be determined 
by the formation of a sharply defined zone on suspended 
strips of paper. 

Expt. 155. Electrolytic flocculation of a gold sol 
—Metals in the colloidal state often show a sudden colour 
change as a first indication of flocculation. Red colloidal 
gold turns blue-violet to blue ; yellow or brown colloidal 
silver changes to red, violet and blue respectively (Expt. 
94). A coarsely disperse flocculation appears in a short 
time as a sequence to the colour changes. 

Determine as in Expt. 153 the concentration of HCl, 
MgCl,, and AlCl, necessary to transform a red gold sol 
into a blue-violet within ten minutes. 

Expt. 156. Electrolytic flocculation of Congo 
rubin—Flocculations accompanied by sudden colour 


COAGULATION AND PEPTIZATION 143 


changes are followed more conveniently with Congo 
rubin than with gold sols, when studying the gold number. 
According to Expt. 97, Congo rubin in a o-or per cent. 
solution suddenly changes to blue-violet with almost all 
electrolytes of certain concentrations. Flocculate the 
Congo-rubin dye solution with baryta or saturated sodium 
hydroxide solution. The colour change and flocculation 
produced by these alkalis is not a chemical change due 
to the liberation of a different coloured acid dye, since 
acids also yield similar results. High hydroxyl ion 
concentrations do not produce this chemical change. 
Nevertheless, the colour change and precipitation of the 
dye take place. 

For quantitative determination of the flocculation 
values as with the As,5; sol, Expt. 153, pipette equal 
volumes, e.g. I c.c. of the dye solution into a series of 
clean test-tubes. Prepare salt mixtures similar to those 
in Expt. 153 and make them up to a volume of 9 c.c. 
Mix the dye with the salt solutions by repeatedly pouring 
the contents of the tubes back and forth. The salt 
concentration which produces flocculation shows a distinct 
colour change toward red-violet or violet-blue after an 
hour. The indication of flocculation by colour transition 
becomes well defined with a little practice. A solution 
for colour comparison may be prepared either by mixing 
methyl violet or azoblue with acid fuchsine or by using 
a Congo-rubin sol at the colour transitions of floccula- 
tion. 

These absolute precipitation values vary with the 
preparation of the dye. Nevertheless, they all show 
a relative difference in flocculating values similar to 
inorganic sols illustrated in the table below :1— 


1¥For further data, see Kolloidchem. Beihefte 12, 94 (1920). 


144 PRACTICAL COLLOID: CHES sa 








Fl , Molar precipitation 

occulation value; jitre of sol floc- 
Electrolyte. in millimols culated by a litre 

per litre. of electrolyte.! 

1 OE on ee Meee SE x. pn 95°9 10°4 

MgCl, ihe 3 hee Gites, ane 1:67 597°7 

PIG Sol i eet spe 0°245 4082: 

Na SO poo ve] = os eee aes 612 16°4 

MesO ole. Vt eee 0°394 2538: 

2 Al,(SO,) 9) +002 7 0°03 33333) 








The effect of the cations on flocculation values is as 
evident here as with the As.S,; sols; while polyvalent 
cations have greater flocculating power, the anion must 
not be neglected. Sulphates flocculate more strongly 
than chlorides. 

Expt. 157. Flocculation of ferric hydroxide sol— 
Use a positively charged ferric hydroxide sol, prepared 
in Expt. 22, or a commercially prepared sol freed from 
excess of chloride ion by warm dialysis. Applying the 
same methods as in the previous experiments, especially 
Expt. 153, determine the flocculation values of NaOH, 
KCl, CaCl,, AICl,, K.SO,, K,-citrate. The following 
data are representative of an experiment performed 
by the author : 

Fe(OH), sol, dialysed. Fe,0,; content = 0-506 per 
cent. 





(a) NaOH. 
1:0 c.c.0-oIN NaOH-+9:0c.c. H,O+10¢.c. Fe(OH), sol=o-5 milli- 
mols 
ALOK eR 3 a +8:0C.Cc, He a ==1-0 
AOC, ie +6:0C.C¢. Re e == 2-O0ls) 
8-0 C.C. ie +2:0C.C. PP * A rUTs 








1 The term “ Molar precipitation in litre of sol flocculated by a 
litre of electrolyte ’’ refers to the number of litres of colloid which 
can be flocculated by a mol of the electrolyte. 


COAGULATION AND PEPTIZATION 145 


oye Ia GIE 
0-5 cc, 2N KCl+-9-5c.c. H,O--10c.c, Fe(OH), sol= 50 millimols 


TOC. 6 » +9:0C.C. ne ee —100 * 
ar OCR, »  —+8:0C.c. * .; — 200 - 
4:0 C.C. » +6:0C.¢. 7 KP == 400 re 
(c) 4 CaCl). 
O*5 c.c. 2N CaCl,--9-5 c.c. H,O-+10c.c. Fe(OH), sol= 50 milli- 
mols 
TO C.C, » +9:0C.C. a bs ==100 - 
2°OC.C. » +8:0C.c. iy ry —200 ig 
4°0 C.C, » +6:0C.c. Pe a = 400 ov 


(d) 4 AICI. 
Pee tele oo cc. 11,.0-toc.c. Fe(OH),sol= 300 milli- 


mols 
4°0 C.C., we t-O°0 C.c. 2 #3 = 600 4, 
8-0 C.Cc. ns +2:0C.c. a Fs —1200 2 

(2) K,S5Q,. 

T:0c.c. 0-oIM K,50,-+9-0c.c. H,O+10c.c, Fe(OH), sol=o0°5 milli- 

mols 
=O C.C, uP +8:0C.C. a Zs =e 1; ) 
BO Cc a +6:0C.c. re Bs == 27s 
80 C.C. ie +2-0C.c. ‘ * =S4°O 3, 


(f) Kg, citrate. 
T:0C.c. 0:005M K, citrate-++9-0c.c. H,O+10¢c.c, Fe(OH); sol= 
0-25 millimols 
+8:-oC.c. es = 
0-5 millimols 
BO GC: ts +6:0C.C, Fe - = 
1-0 millimols 


Z-OE.C, 


Ly} 


Compare these flocculation values with those for 
As.S; (Expt. 153). KCl in both cases flocculated at 
concentrations of the same order of magnitude. On the 
other hand, the flocculation value of bivalent CaCl, and 
especially of trivalent AlCl; are considerably lower with 
As.S; sol; the flocculation value of CaCl, is almost the 
same as that of KCl for Fe(OH), sol, and that of AICI, is 
greater. The flocculation values of K,5O, and potassium 

10 


146 PRACTICAL COLLOID CHEMISTRY 


citrate are of the same order of magnitude as that of 
KCl for As,S;, but a considerably smaller concentration 
is necessary for the Fe(OH), sol. The HCl has a strong 
flocculating activity for the As.S;, while NaOH is par- 
ticularly active for the Fe(OH); sol. Thus, for the 
Fe(OH), sol, the anion determines the flocculation 
values, while the cation determines the flocculation values 
for the As,S;. The behaviour of Congo rubin resembles 
more that of the As.S, sol, yet the simultaneous action 
of both ions of the flocculation electrolyte may be dis- 
tinctly recognized. 

The aluminium hydroxide sol behaves like the ferric 
hydroxide sol. 

Expt. 158. ‘‘ Abnormal series ’’ with mastic sol 
—This term refers to the phenomenon whereby the same 
electrolyte may have a flocculating or non-flocculating 
effect upon a given sol, depending upon its concentration. 
It may be logical to suppose that a dilute flocculating 
electrolyte would have a still greater effect if it were 
added in a larger concentration. However, particularly 
with multivalent electrolytes, this assumption does not 
hold. There is a flocculation range of concentrations 
referred to as the first flocculation zone; then follows a 
range of concentrations in which no flocculation occurs 
referred to as the non-flocculation zone, and finally the 
recurrence of a second flocculation zone. 

Prepare a mastic sol by pouring Io c.c. of a 5 per cent. 
alcohol solution of mastic into go c.c. of water. Dilute 
this concentrated sol ten times (0:05 per cent.) and 
filter. Pipette 5 c.c. of the sol into each of a large number 
of well-cleaned test-tubes. Pour 10 c.c. of a molar 
solution of aluminium chloride into a ten c.c. graduated 
cylinder. Add half of this solution to the first test-tube 
containing the mastic sol and shake the mixture. Fill 
the graduated cylinder with water, halving the concen- 


COaauentiON AND PEPTIZATION 147 


tration of the aluminium chloride and add 5 c.c. to the 
second tube of mastic sol, etc., as illustrated in the table 
below :— 


FLOCCULATION OF 0:05 PER CENT. Mastic HyYDROSOL BY 
AICI, OF VARIOUS CONCENTRATIONS.. 























| AICI, concentra- | AICl, concentra- | 
No.| tion in millimols | Flocculation after || tion in millimols _ Flocculation after 
per litre of 24 hours. ‘| per litre of 24 hours. 
mixture, mixture. | 
I 500 Completely flocculated|| 12 0°25 | Not flocculated 
2 | 250 ” ” 13 OrI25 | By 9 
S| 125 te 5 14 0:064 | Flocculated 
4 64. ” 53 15 | 0°032 | Completely flocculated 
5 2 ? ” 16 | 0-016 Bi) 9) 
6 | 16 ve ¥ 17 | 0°008 | Somewhat turbid 
a 8 Slightly flocculated 18 | 0°004. _ Not flocculated 
8 | 4 Somewhat turbid 19 | 0002 ie s 
com 2 Not flocculated 20 | 0-001 aes a 
IO | I 6 ah pets 0:0005 a, of 
rr | 0°5 ae x } 22 0:00025 |» a 








Beginning with the smallest concentration of AICI; 
solution, and gradually increasing it, the first flocculation 
appears between 0-008 and 0-064 millimolar concentration. 
Then a non-flocculation zone extends to a concentration 
of 4 millimols. Flocculation again appears with an in- 
crease in the concentration of the AIC]; unto the second 
concentration zone.? 

An insight into the theory of this striking phenomenon 
is obtained by an electrophoretic experiment, utilizing 
the ultramicroscopic method first with the sol alone, 
without addition of salt, and then with the sol + AICI, 
at a concentration within the non-flocculation zone. 
The sol possesses a negative charge, while the sol contain- 
ing AICl, has a positive charge in the non-flocculating 
zone.2 The electric charge on the mastic hydrosol may 

1 Tt is understood that on repeating the experiment there will 


be changes in the absolute concentration values. 
2 The positive sol in the non-fluctuating zone appears to give 


148 PRACTICAL COLLOID CHEMIST: 


be neutralized by the addition of AICI, of definite inter- 
mediate concentrations. With smaller AlCl, concen- 
trations, a mastic sol behaves like a negative As,S; sol, 
while with higher AICl, concentrations, it behaves like 
a positive ferric hydroxide sol. 

Expt. 159. Influence of temperature on the 
flocculation of Congo rubin (compare with Expt. 97) 
—Add 5-10 c.c. of a normal KCl solution to 50 c.c. of a 
O-OI per cent. solution of Congo rubin and pour some of 
the mixture into three test-tubes. Place one tube in 
ice water, allow the second to remain at room temperature 
and place the third in a water-bath at about 50° C. 
Flocculation by the electrolyte occurs first at o°C., 
much later at room temperature, and does not take 
place at 50°C. 


REVERSIBILITY OF FLOCCULATION. OF SUSPEN- 
SOIDS 


Most electrolytes precipitate suspensoids irreversibly. 
It is impossible to wash out the electrolyte and again 
change the gel intoa sol. In some cases the irreversibility 
of flocculation is not so much a property of the colloids 
as that of the flocculating electrolyte. Thus, colloidal 
silver, according to S. Odén and E. Ohlon [Zeztschr. fur 
physik. Chem., 82, 78 (1913)], may be reversibly floccu- 
lated by ammonium nitrate. Colloidal sulphur, which 
is classed between typical suspensoids and emulsoids, is 
reversibly flocculated by most alkali salts. The reversi- 
bility of such flocculations may be demonstrated con- 
veniently by Congo rubin, described in Expt. 97. 


up its charge easily. On passage of current no cation migration 
is noticeable in the first 20-30 seconds, but after I-2 minutes the 
direction of migration becomes evident. 


SAE AlION AND PEPTIZATION 149 


The phenomenon of peptization, i.e. washing the gel 
as described in Expts. 30-34, 1s an example of the reversi- 
bility of electrolyte-flocculated suspensoids. 

Expt. 160. Flocculation of suspensoids by dialysis 
—In the preparation of colloid solutions described in the 
iNapter preceding Expt. 30, the presence of a small 
amount of electrolyte is necessary for the stability of 
most suspensoids. If the maximum concentration of 
these “‘ sol-forming’’ ions is exceeded, the colloid floccu- 
lates. Hence, caution must be taken in many cases 
not to carry the dialysis too far. Suitable examples of 
flocculation by dialysis are: first, mercury sulphide 
hydrosols prepared from Hg(CN). (Expt. 14) ; second, 
copper sulphide hydrosols prepared from copper ammo- 
nium hydroxide (Expt. 146); third, cadmium sulphide 
hydrosols (Expt. 3); fourth, positive sols, particularly 
ferric hydroxide sols (Graham). MDialyse 40-50 c.c. of 
the Fe(OH); sol in an analytic dialyser (Expt. 54). 
Compare with the undialysed sol kept at the same tem- 
perature. The Fe(OH), sol flocculates in the dialyser 
after 24 hours. Concentrated Fe(OH); sols form jelly- 
like precipitates. 

Expt. 161. Flocculation by an electric current— 
An electrophoresis experiment continued for a long time 
causes the colloid to flocculate on the electrode to which 
it is attracted. This phenomenon may be clearly ob- 
served by ultramicroscopic electrophoresis. Study the 
electrophoresis of silver and mastic sols and allow the 
electrical contact to last only a few minutes. Coarsely 
disperse, strongly reflecting flocculates may be observed, 
accompanied by the appearance of dark patches, 1.e, 
colloid-free spaces in the field of vision. 

For flocculation by adsorption, see Expt. 146. 


150 PRACTICAL’ COLLOID CHEMI 


B.. FLOCCULATION OF EMUESO RE. 


The flocculation of hydrated emulsoids is characterized 
by the very high concentrations of neutral salts required. 
The reason for this difference in salt concentration is that 
the flocculating electrolyte not only coalesces the particles 
into greater aggregates, but also causes a dehydration of 
the colloid particles, 1.e. a partial separation of the dis- 
persion medium adsorbed by these particles. 

Expt. 162. Qualitative demonstration of suspen- 
soid and emulsoid flocculation—Pour 50 c.c. of an 
As.S; sol into an Erlenmeyer flask. Add 50 c.c. of a 
clear egg-white solution to a second flask. The egg white 
may be prepared by diluting the fresh product 5 times 
with a 0-7 per cent. NaCl solution or by using a 2 per cent. 
solution of dried albumin in a 0-7 per cent NaCl solution. 
Add 5 drops of a saturated solution of ammonium sul- 
phate to the As,S, sol and turbidity results immediately. 
A similar addition to the egg white produces no turbidity. 
A large amount, such as 20—30 c.c. of ammonium sulphate 
solution, will produce turbidity and finally flocculation. 

Certain electrolytes separate the water of hydration 
from the colloid particles. This is a specific property of 
individual salts and for which general rules are not yet 
known. The aggregation and condensation of the 
dehydrated colloid particles in an aqueous dispersion 
medium involve electrical and electrochemical factors. 
The electrical charge plays just as important a role in 
emulsoids as in suspensoids. Electrically neutral albumin 
sols, such as serum albumins, are flocculated by neutral 
salts, alcohol, etc. (Wo. Pault). 

The electrolytic flocculation of albumin sols, previously 
studied, show complex relations. The following general- 
izations have been established. There are two large classes 
of albumin sols, the isostable and isolabile albumin sols. 


COAGULATION AND PEPTIZATION I51 


Isostable albumin sols are stable at the “‘ isolectric points,”’ 
that is, in a state of complete electrical neutrality. 
Serum and egg albumin, hemoglobin, gelatin belong in this 
class. Isolabile albumin sols, when in a state of electrical 
neutrality, are no longer colloidally soluble and hence 
flocculate. Such sols are globulin, casein, stable in weakly 
acid or alkaline solutions. Albumin sols are usually 
amphoteric, that is, they may be either positively or 
negatively charged, depending upon certain conditions. 
They are more easily charged than suspensoids by the 
addition of a small amount of alkali or acid. In alkaline 
media the sol is negatively charged, while in acid media it 
is positively charged. The behaviour of these sols is in 
some respects similar to suspensoid sols. The differences 
between these sols is that the absolute precipitation 
_ values of these emulsoids are smaller; the influence of 
oppositely charged ions is more pronounced and floccula- 
tion is partly reversible. While the neutral emulsoids 
(genuine albumin) show the Hofmeister series, especially 
the cation series; rather indefinitely by flocculation experi- 
ments, the charged protein sols definitely show the series 
(R. Hoéber). The Hofmeister series is reversible, depend- 
ing upon an alkaline (negative) or acid (positive) sol 
medium. 

Expt. 163. Acid and alkaline flocculation of 
casein sol (an isolabile albumin sol)—Add 3-5 g. of 
powdered casein to 100 c.c. of a o-o1N NaOH solution, 
shake the mixture repeatedly, and allow to stand for 24 
hours. The saturated casein sol freed from undissolved 
casein by filtering, shows a very weak alkaline reaction 
towards phenolphthalein. Pipette 2 c.c. of the casein 
sol into a series of test-tubes and determine in the usual 


1 The former protein sol corresponds in behaviour to the sul- 
phide sols, while the serum albumin behaves like silicic acid sol 
in inorganic systems. 


152 PRACTICAL COLLOlUMD CHE ane 


manner the concentration of HCl and NaOH at which 
the sol becomes turbid or flocculates. Start with o-o1N 
HCl and gradually decrease the concentration. ‘The first 
acid flocculation of the sol occurs with a mixture of 2 c.c. 
of casein sol and 3:0 c.c. o0-o1N HCl+ 5 c.c. HO. This 
acid flocculation value is equivalent to about 0-0025 mols. 
The isolectric point is attained at this concentration. 
According to L. Michaelis, the (H*)-ion concentration of 
the isolectric point is equal to 2-4 xX I0°. A second 
flocculation occurs with higher acid and alkali concentra- 
tions. The flocculating concentrations for HCl is about 
0-25 mols, 1.e. about 100 times greater concentration of 
acid than that required for the first flocculation point, 
and for NaOH it is about 5 mols. These flocculation 
values may be determined more accurately by starting 
with N HCl in one case and with 8N NaOH in the other. 

Expt. 164. Neutral salt flocculation of hamo- 
globin (an isostable albumin sol)!—Use the powdered 
preparation ; if the hemoglobin is in the form of lamelle, 
grind it in a mortar before use. Dissolve 2 g. in 100 c.c. 
of water by first grinding the powder with a little water 
in the mortar in order to lessen lump formation. A 
better method is to sift the powder by brushing it through 
a wire screen into a beaker containing water constantly 
stirred. Filter the solution and proceed as in the pre- 
viously described flocculation experiments by pipetting 


1 Jn the experience of the author, coagulation experiments on 
hemoglobin are particularly suited for this important chapter on 
the colloid chemistry of proteins. The material is easily obtain- 
able in uniform composition ; it gives relatively clear solutions 
of greater concentration than serum or egg albumin; it may be 
dissolved in any desired concentration and acidified or made 
faintly alkaline without inducing coagulation. The flocculation 
value is relatively low, so that working with extremely concen- 
trated acid solutions or with salts is obviated. The flocculation 
value may be determined with great accuracy. 


COAGULATION AND PEPTIZATION 153 


2 c.c. of the hemoglobin solution into a series of test- 
tubes. Prepare the salt mixtures in another series of 
tubes and make them up to a volume of 8c.c. Thorough 
mixing may be obtained by pouring the added solutions 
back and forth several times. The point at which a 
distinct turbidity is observed immediately after mixing 
may be taken as the flocculation concentration. Tur- 
bidity is recognized by comparing with a control tube. 
Determine the flocculation values of a number of elec- 
trolytes on the neutral, weakly alkaline and weakly acid 
hemoglobin solutions. The following example gives the 
approximate flocculation values usually obtained :1 


I. ELECTROLYTIC FLOCCULATION OF HA#MOGLOBIN 
K,-citrate, 2N = 0-66 mols. 
2c.c. hem. + [2 c.c. K,-citrate + 6c.c. H,O] 
= 0-134 molar 
2c.c. hem. + [4 c.c. K,-citrate + 4c.c. H,O] 
= 0:267 molar 


Ke 20; IN = 0°5 molar. 


2 c.c. hem. + [4c¢.c. K,SO,+ 4 ¢c.c. H,O] = 0-2 molar 
2c.c. hem. + [8 c.c. K,SO, + 0c.c. HO] = 0-4 molar 


Keacetate, 2N = 2 molar. 


Zeoicelizem, ——|2c.c. K-acetate + 6 c.c. H,O] 

== 0-4 molar 
2c.c. hem. + [4 c.c. K-acetate + 4c.c. H,O] 

— 0:8 molar 


1 Jn the author’s knowledge, previous investigations on the 
neutral salt flocculation of hemoglobin are as yet not available. 
In the above experiment the heavy-typed figures are only approxi- 
mate flocculation values, 


154 PRACTICAL |\COLLOID CH ii ana 


KG] ANI ==aamolatie= saturaveds 

2c.c. hem. + [4 c.c. KCl1+4c.c. H,O] = 1-6 molar 

2-c.c. hem. + [6 c.c. KCl + 2 c.c) HO} 2-4maias 

2c.c. hem. + [8 c.c. KCl-+ 0 c.c. H2O) == 3-2 molar 
KNO,, 4N = 4 molar = saturated. 

2c.c. hem. + 8c.c. KNO,; = 3-2 molar. 

Flocculation value > 3-2 molar 

KCNS, saturated — ca. 14 molar ; no flocculation. 


For three additional sulphates and chloridés, the follow- 
ing flocculation values are shown in a similar manner : 


(NH,).5O, = 0-09 molar CaCl, = 0-004 molar 
NasoQy O40 i. MgCl, = 0-004 us 
TH35O" == DO aS AlCl, 7-33 2s 


Arrange in series the flocculation values obtained for 
_ the potassium salts: citrate, sulphate, acetate, chloride, 
nitrate, sulphocyanide. The cations with the sulphates 
give the series: NH,, K, Na, Li. The chlorides orithe 
alkaline earths show extraordinarily small flocculation 
values. 3 


II. FLOCCULATION OF ELECTRONEGATIVE HAMOGLOBIN 


Final concentration = 0:03N NaOH. 

2 cc. hem.+ [2 cc. saturated’ (capes 
(NH,).50,-++ 6 cc. H,O+6 drops 1N NaOH] = 0°8 - 
molar, immediate flocculation. 

2 .c.c. hem,-+ [8 c.c. saturated’ Ca- escuela 
NH,CNS +6 drops IN NaOH]=ca. 6 molar; no 
flocculation. 


III. FLOCCULATION OF ELECTROPOSITIVE HA#MOGLOBIN 


2c.c. hem. + [2 c.c. molar (NH,).50,-+ 6 c.c. H,O 
-+- 6 drops NH,Cl] = 0-2 molar ; immediate flocculation. 


COAGULATION AND PEPTIZATION 155 


eee ening (2 C.c. 0-2 molar NH,CNS-+ 6 c.c. 
H,O + 6 drops NHC1] = 0-04 molar ; immediate floccu- 
lation. } 

The experiments with alkaline and acid hemoglobin 
give the following flocculation values: Negative sols— 
sulphate, 0:8; sulphocyanide, >6-0. Positive sols— 
sulphate, 0-8; sulphocyanide, 0-04. The sulphate floc- 
culates negative sols more readily than sulphocyanide. 
However, the sulphocyanide reacts more strongly than 
the sulphate toward positive sols. These results show a 
reversal of the Hofmeister series, depending on the sign 
of the charged sols. Compare the behaviour of As.S; and 
Fe(OH), [Expts. 140 and 141]. Determine the floccula- 
tion values of the above series, using 0:03N alkaline and 
acid solutions. 

The following table is a summary of the flocculation 
values found by F. Hofmeister for egg white with 
potassium and sodium salts 1 :— 


FLOCCULATION VALUES. 











Hemoglobin Egg white 
(K-salts). (Na-salts). 
Mols per litre. | Mols per litre. 
Ri ceee eres SC. 0:27 | 0°56 
PeRPIT AONE R EE ee ke —— 0:78 
DOIALGnee eR we 0-4 | 0:80 
Nhs rr 0:8 1°69 
(CUD ie TS) ee 2°4 3°62 
OLGA ec ae | 5°42 
Gy RRS Eee os a — | 5°52 
TOUCHE Sl Ca. 5 very large 
Bilpnocyanidg. 6... = 1A | very large 











1 Cited after R. Héber, Physical Chemistry of Cells, 4th Edition, 
I914, p. 308. 


156 PRACTICAL COLLOID, CHEMISi i= 


Determine the cation series, SO,> as anion, with 
alkaline and acid hemoglobin in the same way. The 
series obtained is as follows : 

Alkaline Li* (1 mol} ~>NH,* (0°6)) > Kass) 

Acid Lit (0-13 mols) — NH,? (0-04) — K® (0-025). 

A greater sensitivity is shown by the acid sols, yet the 
cation series remains the same with the alkali and acid 
concentrations used. This does not apply to other con- 
centrations of acid or alkali. 

Concentrated hemoglobin solutions are also flocculated 
by additions of acid or alkali mixed with neutral salts. 

Expt. 165. ‘* Irregular series ’’ with dialysed eg¢ 
white—Use an albumin sol freed from globulin and salts 
by dialysis. Prepare by the above procedure 2 c.c. of 
albumin and 8 c.c. of aqueous salt solution, using molal 
lead nitrate as follows : 


2c.c. albumin + 8c.c. M Pb(NO;). = 0-8 molar. 
2c.c. albumin + [4 c.c. M Pb(NO;).+ 4 c.c. H.O] 
—'O-4 TiGlateetc 


An experiment in which an old dialysed preparation 
was used, gave the following results after two hours : 


o-8 molar * strong flocculation) 6-4 milliniols ) 





a oa +Clear 
0-2 7 1°60 ye weakly turbid 
O'l - turbid 0:8 y turbid 
105 ater ) 04 7 weakly turbid 
0:025 -clear 0-2 

; clear 
0:0125 ,, ) orl 


Expt. 166. Influence of temperature on the elec- 
trolytic flocculation of gelatin solutions—Add 
enough saturated ammonium sulphate solution to 50 c.c. 
of 0-5-I per cent. aqueous gelatin at room temperature 
until the first appearance of a faint turbidity. Clear the 
solution by adding a few drops of water. Pour the 


COAGULATION AND PEPTIZATION 157 


mixture into three test-tubes. Place the first in an oven, 
the second in an ice-chest and allow the third to remain 
at room temperature. After 24 hours, the solution at the 
higher temperature has remained clear. A faint tur- 
bidity appears in the one at room temperature and a 
strong turbidity or flocculation in the tube which was 
placed in the ice-chest. If solutions 2 and 3 are warmed, 
the flaky precipitate dissolves to produce a slightly turbid 
liquid, which precipitates again on cooling. Compare 
with Expt. 159 on Congo rubin. 

Expt. 167. Flocculation of hydrated globulin by 
electrolytic extraction—Egg white, next to albumin, 
contains considerable globulin. Globulin, like many sus- 
pensoids, is colloidally soluble in the presence of certain 
small amounts of electrolyte. Not only (H*™) and 
(OH) ions, but particularly neutral salts have a dispers- 
ing action upon globulin. Prepare the following mixtures 
of natural egg white and distilled water : 


Seeccrces white 5  c.c. H,O 


em. Grins ta 7 5 COs, 
Eee Ce 64 phe 75 CC. yy 
G20 GAC? 4; SE gt ele by Raa Ok Ce 


Observe that increasing dilution produces a constantly 
increasing turbidity, and if the solution is diluted ten 
times, practically all the globulin precipitates. 

Pour a few c.c. of fresh clear egg white into an analytic 
dialyser enclosed in a vessel. Guard against bacterial 
growth by adding chloroform. Dialyse for 1-2 days and 
change the water often. Large globulin aggregates 
appear within the dialyser. Compare analogous experi- 
ments with Fe(OH), (Expt. 160). 

Expt. 168. Reversible and irreversible electrolytic 
flocculation of eg white—Flocculate a mixture of 
2 c.c. of egg white and 8 c.c. of salt solution with 


158 PRACTICAL COLLOID CHEMISTRS 


ammonium sulphate (about one molar) and with calcium 
chloride. Allow the precipitate to settle, wash by décanta- 
tion with distilled water or pour a few drops of the turbid 
mixture into a beaker containing distilled water. The 
ammonium sulphate precipitate redissolves, while barium, 
calclum and _ strontium salts, but not magnesium 
salts, produce irreversible flocculation (Wo. Pauli). 
Analogous to the suspensoid flocculation, reversibility or 
_ irreversibility of the process depends less upon the nature 
of the colloid and more upon the flocculating medium. 

If the albumin receives a charge by addition of acid or 
alkali, then the flocculation by ordinary alkali salts be- 
comes irreversible. Repeat the above experiment with 
ammonium sulphate, using a faintly acid and faintly 
alkaline albumin successively and observe that on longer 
standing, the flocculation becomes increasingly irrever- 
sible. 

Expt. 169. Alcohol flocculation of hemoglobin— 
Determine the flocculation value of ethyl alcohol upon 
neutral hemoglobin. Use 2 c.c. of 2 per cent. hemo- 
globin and 8 c.c. of alcohol-water mixture. The floccula- 
tion values are usually between 20 per cent. and 40 per 
cent. by volume of alcohol. Perform the same experi- 
ment with weakly acid (0-03N) and weakly alkaline 
(0-03N) hemoglobin. Observe that the electrically 
charged hemoglobin, which is strongly hydrated, may 
be flocculated by a rather high alcohol concentration. 


COAGULATION OF. DIALYSED EGG WHITE BY HEAT 


The coagulation of albuminous substances by heat 
involves chemical changes of denaturization which accom- 
pany phenomena of flocculation. The chemical and 
colloidal processes may be differentiated from one another 
in such a way that under certain conditions the albumin 


COAGULATION AND PEPTIZATION 159 


may be denaturized by heating without any flocculation 
resulting. However, the colloidal process of flocculation 
may be produced by cooling (Wo. Pauli and H. Handow- 
sky). 

Expt. 170. Coagulation of dialysed egg white plus 
KCNS by heat—Dialyse an egg-white solution from 
globulin and salts.‘ To the dialysed egg white add 
sufficient potassium sulphocyanide to make the solution 
approximately 2N and boil a few minutes. Allow to 
cool, pour half the mixture into an analytic dialyser and 
change the wash water frequently during the first hour. 
Usually, a strong turbidity appears in the dialyser after a 
few hours, while the undialysed mixture has remained 
clear. The same results may be obtained by using KI 
instead of KCNS. Denaturization rather than flocculation 
results in the presence of sulphocyanide. Flocculation 
may be produced by the removal of this salt. This is 
analogous to the flocculation of globulin by dialysis of 
natural egg white. | 

Expt.171. Influence of electrolytes on the coagula- 
tion temperature of dialysed egg white—The simplest 
method for the determination of coagulation temperature 
is the optical method based upon the appearance of 
turbidity (see page 132). The coagulation temperature 
depends upon the rate of heating, as in the experiments 
upon gelation, corresponding to Expt. 108. A _ larger 
“normal’”’ rate of temperature change, 1° C. per minute, 
is advisable for this experiment. The solutions are con- 
veniently heated in a small test-tube placed in a beaker 
of water. The results may be reproduced accurately to 
at least half a degree, after a few trials. 

Determine the coagulation temperature in the presence 
of neutral salts, such as the potassium salt used in Expt. 


1 The above experiment cannot be performed with non-dialysed 
globulin containing egg white. 


160 PRACTICAL CGLLOID CHEMIST 


153, with a final concentration of 0-5, 0:25 and o-I25N. 
The following example gives approximate values !: 


no addition 60° nitrate 65° 
citrate Thee bromide 62553 
acetate 70 iodide 60° 
chloride oy sulphocyanide 60° 


Observe that all salts which exert any influence raise 
the coagulation temperature (Wo. Pauli).2 Furthermore, 
the Hofmeister series appears again. R. H6ber finds a 
reversal of the Hofmeister series on using similar concen- 
trations with dialysed egg white 3: 


acetate 62:0 bromide 66-5 
_ chloride 62-0 iodide 76:5 
nitrate 66-4 sulphocyanide fe 


Other salt concentrations produce an irregularity in the 
series so that with egg white and o:15N mixtures a 
reversal of the ionic series occurs (R. Héber). Using 
concentrations of iodide, cyanide, sulphocyanide, etc., 
and raising the temperature produces no flocculation. 

Expt. 172. Theory of emulsoid precipitation— 
Emulsoids are liquid-liquid systems. The following 
experiment proves the applicability of this definition 
-(K. Spiro, Wo. Pauli). Mix a hot 5 per cent. gelatin 
solution with sufficient powdered sodium sulphate, such 
as 2-2:5 M solution, to form a milky flocculate. Place 
the test-tube containing the gelatin-salt mixture upright 
in an incubator or water-bath. Heat at a temperature 
of 35°-50° C. for 24 hours. The precipitate settles, but 
on account of its high water content at the above tem- 
perature it flows into a completely coherent yellow layer, 


1 An old preparation preserved in toluol was used. 

2 All salts decrease the turbidity of gelatin solutions according 
tO REx pina: 

’R. Héber, Hofmeister’s Betiydgem i ies eo 


COAGULATION AND PEPTIZATION _ 161 


which is often clear and at least transparent at the 
edges. The precipitate behaves like a fluid when the 
vessel is tipped. The disperse phase is still a liquid 
under the experimental conditions described. 

A liquid gel may be obtained more rapidly if a 1-2 
per cent. gelatin solution is flocculated with 0:5N salicylic 
acid and allowed to stand for 30 minutes at 35°—40° C. 
The precipitate, depending upon the amount flocculated, 
settles to the bottom of the tube in the form of drops or 
as a coherent phase. 

There is no doubt that a part of the neutral salt action 
consists in a dehydration of the colloid particles. There- 
fore the disperse phase of the colloid in the above ex- 
periment is richer in water when uncoagulated and must 
have the properties of liquid drops. 


C. OPPOSING INFLUENCE OF COLLOIDAL 
SOLUTIONS 


Ly -BLOCCULATION OF TWO COLLOIDS 


Hardy’s rule states that oppositely charged colloid 
particles flocculate each other. This is also true of 
coarsely disperse, colloidal, or molecularly disperse par- 
ticles of opposite charges. Flocculation of two oppositely 
charged colloids may be simultaneously produced by 
mixing them. This type of flocculation is characterized 
by the fact that it may occur only when the ratio of the 
concentrations of the two colloidal solutions lies within 
certain narrow limits. Often many trials must be made 
in performing such experiments before optimum floccu- 
lation concentrations are found. Such concentrations 
produce a completely clear supernatant liquid, due to 
complete flocculation. Prepare a dozen mixtures of night 
blue and Congo red at optional concentrations and record 

II 


162 PRACTICAL COLLOID, CHEAIS iia 


the proportions. A systematic procedure must be used 
for the determination of flocculation optima. 

Reciprocal flocculation of colloids may often be regarded 
as reciprocal adsorptions. The precipitates formed are 
a particularly important class of adsorption compounds. 
These precipitates differ from ordinary chemical precipi- 
tates in that their components are not necessarily com- 
bined in stochiometrical proportions (compare Expts. 
178 and 179). 

Expt. 173. Reciprocal flocculation of arsenic 
trisulphide and ferric hydroxide sols—Prepare an 
As. Sol from 0-5 per cent. As.O,; according to Expt. 153 
and dialysed ferric hydroxide by Graham’s method. 
The flocculation optimum may easily be found when 
using such sol mixtures. The following table gives a 
personally conducted experiment. The results were 
noted after 24 hours. 

As.S, sol; content -0-5-per cent, referredsipe se. 

Fe(OH), sol; content 0-5 per cent. referred to Fe,O3;. 


i cc. As.$5;-+ 9 c.c. Fe(OH \se icleameiram 


Tidy Pr Sa. ep eas - faintly turbid 

550 ” ” + 5°0 ” o turbid 

Obs ay 19 0 poe eae a precipitate, turbid, 
brown super- 
natant liquid 

A) ee So tO ee, . completely floccu- 


lated, clear col- 
ourless super- 
natant liquid 


Seay ton » + Io drops a precipitate, turbid, 
yellow super- 
natant liquid 

Toa; Pincers 5 : 4 precipitate, faintly 


turbid, yellow 
supernatant liquid 


COAGULATION AND PEPTIZATION 163 


Pees, 2 c.c. Fe(OH), — fine precipitate 
"3 eee he est 1 “ni , clear yellow. 


Expt. 174. Reciprocal flocculation of Congo red ! 
and night blue (Buxton and Teague)—Prepare the 
following four solutions of Congo red and the following 
eight solutions of night blue, starting from I per cent. 
solutions : 

Congo red: 0:0125, 0:0100, 0:0083, 0:0063 per cent. 
Night blue: 0-0333, 0°:0250, 0:0200, 0:0167 per cent. 
0:0125, O-0100, 0:0083, 0:0063 per cent. 

5 c.c. of Congo red are mixed with 5 c.c. of night blue 

according to the following scheme :— 








Congo red. Night blue. 

O°0125 0:0333 0°0250 0°0200 0O-0167 00125 
100 0:0250 0°0200 0-0167 0:0125 0-0I00 
83 0°0200 O:0167 O°0125 O:0I00 0:0083 
63 0:0167 0°0125 o-0100 0:0083 0:0063 





In an example personally conducted, complete floccu- 
lation resulted and a colourless supernatant liquid 
appeared. The optimum quantities necessary for floccu- 
lation usually vary with the salt content of the individual 
sols. 

Expt. 175. Reciprocal titration of two dyes (L. 
Pelet-Jolivet)—The previous experiment may be modi- 
fied and performed quicker by using Tupfel’s method. 
Place a drop of both dyes mixed in proportions insuff- 
cient for complete flocculation upon filter paper. The 
dye present in excess forms a “mirror.” If the Congo 
red is in excess, the mirror or its edge is red. If the night 
blue is in excess, the edge is blue. Two reciprocally 


1 Not to be confused with Congo rubin. 


164 PRACTICAL COLLOID CHEMIS2Ra 


flocculating dyes may be titrated by the Ttipfel method 
described by L. Pelet-Jolivet, The Theory of Dyeing, p. 49 
(Dresden, 1910). The following experiment conducted 
by the author is illustrative : 

5 c.c. O-OL per cent. Congo red titrated with 0-033 per 
cent. night blue. 


3-0 c.c. night blue, red mirror. 


5000, - same, but weaker. 
6-0 C.c. : much weaker. 
6-276.) e indifferent mirror. 
O57e:C :, faint blue mirror. 
Oe} he decided blue mirror. 


The proportional amounts in this example are 6-2 
parts of night blue to 5 parts Congo red, equal to 1:24. 
When titrating 5 c.c. of 0-0063 per cent. Congo red with 
0:33 per cent. night blue, the proportions amount to 61 
parts of night blue to 38 parts Congo red or1-:22. Ina 
great number of experiments carried out in the laboratory, 
5 c.c. of 0-01 per cent. Congo red, titrated with o-or per 
cent. night blue in a porcelain dish with a glass rod, gave 
a proportion 1:2 to 1-3. When titrating with more 
dilute mixtures, a value of about two is obtained. The 
amounts by weight necessary for complete reciprocal 
adsorption are independent of the concentration of the 
reaction mixture. A convenient pair of dyes is methylene 
blue and crystal ponceau. 


II. PROTECTIVE ACTION 


M. Faraday discovered that small amounts of solvated 
emulsoids bestow a considerably greater stability upon 
suspensoids toward the flocculating action of electrolytes. 
The action of such protective colloids was already men- 


DOASUEATION AND PEPTIZATION 1365 


tioned in the preparation of colloidal solutions (Expts. 
45 and 46). Their mode of action is not due to an in- 
crease in the viscosity of the dispersion medium, for in 
many cases very small amounts prove effective. There 
appears to be a union between the suspensoid and emul- 
soid particles, with the result that the relative stability 
of the protective colloid is decisive for the whole complex. 
As yet it is not known if there is a 


ce 


coating’’ or “ en- 
veloping ”’ of the suspensoid particle by the liquid drops 
of the protective colloid. 

Such protective colloids are gelatin, isinglass, albumin, 
casein, hemoglobin, tragacanth, acid and _ alkaline 
hydrolysis products of egg white, lysalbin and protalbin 
acids, tannin, etc. Freshly prepared stannic acid is a 
protective inorganic colloid. Related material in regard 
to organic protective colloids may be obtained from the 
studies of A. Gutbier and his students in the Kolloid. 
Zeitschrift, IQlO-1g22. 

Expt. 176. Gold numbers (R. Zsigmondy)—Use 
an electrolyte-sensitive red gold sol, prepared in Expt. 3 
with alcohol. [R. Zsigmondy, Colloid Chemistry, 2nd 
edition, 1918, p. 174; Zettschr. f. analyt. Chem., 40, 
Oooo ty iat ace) O01, 0-1, 1°, etc., c.c. of the protective 
colloid to be studied in a series of small beakers with 
just Io c.c. of a red electrolyte-sensitive gold sol. After 
3 minutes, pour I c.c. of a ro per cent. solution of NaCl 
into each beaker, with constant shaking. By systematic 
decreasing of the limits of concentration determine which 
concentration of protective colloid is just sufficient to 
prevent the sudden colour change from red to blue. 
These numbers expressed according to R. Zsigmondy in 
mg. of protective colloid, may be more conveniently 
expressed in per cent. and are known as the “ gold 
numbers ’”’ of the protective colloids used. The orders 
of magnitude are: 


166 PRACTICAL COLLOID "CHEMiSeta a. 


Gelatin . : . 0:00005—0-0001 per cent. 
Oxyhzemoglobin . _0°0003-0-0007 per cent. 
Sodium caseinate O-OOOI per cent. 
Albumin . ; 0-OOI—0:002 per cent. 
Staroh wee . ..€a.70°25 pencens 


The gold numbers give a quantitative estimate of the 
protective power of various emulsoids. They may only 
be taken as relative and not as absolute values because 
their numerical values vary not only according to the 
nature of the gold sol, such as degree of dispersion, con- 
centration, mode of preparation, etc., but also with the 
colloidal nature of the protective colloid. Determine 
the silver number in a similar manner by using a brown- 
red sol, prepared in Expt. 10, and choose for the end point 
its sudden change to grey-violet. Determine the As,.S, 
number by assuming at the end point the appearance of 
turbidity upon mixing. 

Expt. 177. ‘*Congo-rubin numbers ’’—Congo 
rubin is also suitable for the quantitative study of pro- 
tective.action and may be used as a gold sol substitute. 
Start with a 1 per cent. dye solution and pipette I c.c. 
into small test-tubes or beakers. Add various amounts 
of the protective colloid solution to the Congo rubin, 
make up to a volume of 5 c.c. with water and add to 
each mixture 5 c.c. of 0:-5N KCl. Determine the concen- 
tration of protective colloid, which produces a difference 
in colour shade after ten minutes. Compare with a 
control solution containing KCl of the same concentration. 
The following ‘‘ Congo rubin numbers””’ are illustrative : 


Sodium caseinate 0-004 per cent. 
Hemoglobin . ; 0-008 per cent. 
Albumin . 0-020 per cent. 
Gelatin . : 0-025 per cent. 
Soluble starch . ca, O-L per cent: 


bec Lames ; ; : ca. 0-2 per cent. 


COAGULATION AND. PEPTIZATION 167 


Expt. 178. Cassius purple—Add a few c.c. of a 
O-OI percent. solution of stannous chloride to a 0:05 gold 
chloride solution. A brown to a beautiful purple-red 
colour first appears and the sol flocculates upon addition 
of any neutral salt. This so-called Cassius purple is an 
“ adsorption’? compound of colloidal gold and colloidal 
stannic acid. Such a composition was predicted by M. 
Faraday. As in the preparation of tannin gold (Expt. 
2), the addition of stannous chloride acts in two ways: 
(1) It produces colloidal gold by reduction; (2) the 
colloidal stannic acid, formed by hydrolysis at such a 
dilution, acts as a protective colloid. 

The correctness of this assertion is shown by the fact, 
as pointed out by R. Zsigmondy, that if separately 
prepared solutions of colloidal gold and stannic acid are 
mixed, the resulting mixture behaves like Cassius purple. 
Add to a red gold sol a stannic acid sol prepared from 
stannous chloride, according to Expt. 39, and then a 
neutral salt to the solution. Compare with a similar 
experiment, using a pure gold sol. The gold-stannic 
acid mixture does not change suddenly to blue-violet. 
This mixture illustrates the colloidal reaction of stannic 
acid sols, since a coarse rather than a fine red precipitate 
is formed. Old preparations of stannic acid sometimes 
show a rather weak protective action. 

Analogous adsorption compounds may be prepared 
with colloidal silver or platinum and stannic acid. 

Expt. 179. Rubin purple—tThe protective action of 
stannic acid may be shown with Congo rubin as well as 
with gold sol. Add 2-3 c.c. of a stannic acid sol, prepared 
in Expt. 39, to 10 c.c. of a o-or per cent. Congo-rubin 
solution, freshly prepared with CO, free distilled water. 
Add the same amount of water to a control solution. 


1 The amounts added vary with the concentration of the stannic 
acid sol and can be determined in advance. 


168 PRACTICAL COLLOID (CHENS ii 


The control suddenly changes to a deep blue or violet in 
afew seconds. Addition of neutral salts causes the most 
rapid and complete change, especially upon addition of 
a few drops of o-‘o1N aluminium sulphate. The sol 
protected with stannic acid remains red. It is difficult 
to prepare mixtures containing a strong excess of salt 
which will remain distinctly red after standing a few hours, 
for a red precipitate gradually separates out. The 
precipitate, rubin purple, is an anologue to Cassius 
purple. 


D, PEPTIZATION 


Peptization is the reverse of coagulation. It involves 
a change of a coarsely disperse precipitate into the 
colloidal state. Examples of peptization were given in 
Fxpts. 30-39. Inthesimplest cases the precipitate spon- 
taneously decomposes to form the colloidal solution. In 
other cases the precipitate may be changed into a colloid 
by dilution or washing. This is known as reversible 
colloidal solubility and has already been mentioned 
in the paragraph preceding Expt. 30. Such examples 
represent peptization processes in a restricted sense and 
generally consist in the treatment of precipitates with 
electrolyte solutions. Examples were given in Expts. 
30-39 ; other peptization processes are described in the 
following experiments. 

Expt. 180. Peptization phenomena—Flocculate a 
ferric hydroxide sol prepared by the Graham method 
with potassium citrate by first preparing a whole series 
of concentrations in order to determine the flocculation 
optimum.! Decant or wash the gel by centrifuging and 
mix the gel with a little ammonium hydroxide to change 
it into a colloidal solution. 

1 The flocculation of ferric hydroxide sol with citrate gives an 
irregular series, 


GORGULATION AND PEPTIZATION 169 


Flocculate a large amount of silver sol with ammonium 
nitrate and wash the precipitate as above. Suspend it 
in distilled water to which a trace of NH,OH has been 
added. ‘The precipitate regains its colloidal state, giving 
a clear brownish stable gel. Wash the purple of Cassius 
obtained by precipitating a red gold sol with potassium 
chloride and suspend it in water. Upon addition of 
small amounts of NH,OH, a colloidal solution is obtained. 
The same experiment may be performed with rubin 
purple. 

Colloidally disperse substances are acted on by the 
same reagents that combine chemically with the sub- 
stances in a coarser state. Hence, chemical changes 
in colloid solutions which lead to molecular dispersion 
are called dissolutions. It appears, however, that such 
dissolution is modified in some respects by the colloid 
state. 

Expt. 181. Dissolution of red gold sols by potas - 
sium cyanide (C. Paal)—Add a few drops of 2N 
potassium cyanide solution to a gold sol. The gold, upon 
gentle warming, instantaneously decolorizes or will do so 
within five minutes at room temperature. 

The experiment at the same time furnishes an example 
of the increased rate of reaction of colloid systems 
according to the so-called Wenzel law. If a larger piece 
of gold is left in contact with KCN, a small amount 
dissolves after some time ; hence the rate of dissolution 
is slower. 

Expt. 182. Behaviour of silver sols toward nitric 
acid—Mix a suitably concentrated brown-red silver 
sol with a few drops of nitric acid. The sol changes to 
a grey-violet or black and then flocculates. It gradually 
dissolves after continued shaking and standing. 

Expt. 183. Coagulation and dissolution of silver 
bromide sols by ammonium hydroxide (R. Auerbach) 


170 PRACTICAL COLLOID CHEMIS tras 


—Prepare a fresh silver bromide sol in the following way : 
Add 12 c.c. of o-IN KBr to 80 c.c. of distilled water and 
add 8 c.c. of o-IN AgNO;solution. Pour 10 c.c. into four 
test-tubes, add the following quantities of solution and 
stir : 
Tube zr. zo c.c. distilled water. 
» 2 2:5 0¢.c.2N NH,OH + 25 c.c. distilled water. 
pyr ti LONER. SINGIN LE le 
» 4. o.c.c. 4N or stronger NOE. 


Tube I serves as a control. Immediately after the 
addition to tube 2 a stronger turbidity appears. The 
solution in tube 3 increases in turbidity and then becomes 
clear. Dissolution takes place instantaneously in tube 4. 

The question whether a sol first flocculates upon addition 
of a dissolving electrolyte or is directly dissolved is 
obviously answered by comparing the rates of both 
processes. The rate of flocculation of silver sol is greater 
than that of dissolution. Mix an As.S; sol with NaOH 
or a positive Fe(OH); sol with a little HCl. The dis- 
solution process proceeds so rapidly that flocculation by 
this addition of electrolyte is apparently impossible, at 
least it cannot be observed. The addition of larger 
amounts of HCl to a Fe(OH), sol first causes a flocculation. 
The silver bromide experiment illustrates these three 
possibilities. 


IX 
COMMERCIAL COLLOIDS 


4 NHERE are numerous commercial ‘“ natural ’’ 
colloids. Hydrated emulsoids are, as a rule, 
obtainable as solid resoluble gels. They may 

be used for colloid chemistry experiments in this state, 
as well as in the disperse form, in a suitable dispersion 
medium. Suspensoids are likewise prepared in a solid 
resoluble form. The sols made by electrical methods are 
obtainable in solution. 


A. INORGANIC COMMERCIAL COLLOIDS 
Me LAL COLLOLDS! 
GOLD 


Colloidal gold—Dark red glistening lamelle. Colour 
of solution : reddish black, metallic ; purple red in trans- 
mitted light. Au content, about 75 per cent. 

Electro-colloidal gold solution—cColour of solution : 
reddish black in reflected light ; violet red in transmitted 
light. Au content, about 0-03 per cent. 

Colloidal gold solution—Colour of solution: dark 


1 The number of colloid particles is proportional to the strength 
of the current. The base metals formed in aqueous solution 
undoubtedly have oxidation products in addition to the metallic 
element. 


it 


172 PRACTICAL COLLOID CHEM ae 


red in reflected light ; purple red in transmitted light. 
Au content, about 0-005 per cent. 


PLATINUM, PALLADIUM 


Colloidal platinum—Black glistening lamelle. 
Colour of solution: deep black in reflected light ; deep 
brown in transmitted light. Pt content, about 60 per 
cent. 

Electro-colloidal platinum solution—Colour of 
solution: black in reflected light ; dark brown in trans- 
mitted light. Pt content, about 0-04 per cent. 

Electro-colloidal palladium—Colour of solution : 
greenish brown. Pd content, about 0-08 per cent. 


SILVER 


Collargol—Metallic glistening, brown green lamelle. 
Colour of solution: black brown in reflected light ; dark 
brown in transmitted light. Ag content, about 75 per 
Gert. ; 

Electro-collargol—Colour of solution: black brown 
in reflected hght ; dark brown in transmitted light. Ag 
content, 0:06 per cent. 

Electro - collargol, © concentrated — Ten times 
stronger than the previous solution. Colour of solution : 
deep black in reflected light ; dark brown in transmitted 
light. Ag content, 0-6 per cent. 

Skiargan—A ro per cent. sterile solution of a stable 
go per cent. colloidal silver. It is used for R6ntgen 
diagnosis, especially in pyelography. Ag content, 9 per 
cent. 

Choleval is a colloidal silver with gallic acid salts as 
a protective colloid. 


COMMERCIAL COLLOIDS ses 


MERCURY 


Colloidal mercury—Heavy grey black; external 
surface of particles show metallic lustre. Colour of 
solution: grey black in reflected light ; deep brown in 
transmitted light. Hg content, about 7 per cent. 


Electro-colloidal mercury—Solid, grey _ black, 
shining, heavy particles. Colour of solution: grey in 
reflected light; light grey brown in transmitted light. 
Hg content, about 55 per cent. 


Electro-colloidal mercury solution—Colour of 


solution: grey in reflected light ; brown in transmitted 
light. Hg content, about 0-09 per cent. 


COPPER 


Electro-colloidal copper—Colour of solution: black 
in reflected light ; dark reddish brown in transmitted 
leh eweiecOntctt, about 0°22 per cent. 


ARSENIC 


Colloidal arsenic—Blue black glistening lamelle. 
Colour of solution: reddish brown to grey in reflected 
light ; dark reddish brown in transmitted light. As 
content, about 33 per cent. 


ANTIMONY 


Colloidal antimony—Black — glistening lamelle. 
Colour of solution: grey black in reflected light ; dark 
reddish brown in transmitted light. Sb content, about 
20, per cent, 


174 PRACTICAL COLLOID CHEMTsa as 


VANADIUM 


Colloidal vanadium—Colour of solution: grey 
black in reflected light; greenish grey in transmitted 
light. V content, about 0-07 per cent. 


TIN 


Electro-colloidal tin—Colour of solution: grey 
black in reflected light ; grey brown in transmitted light. 
Sn content, about 0-3 per cent. 


TITANIUM 


Electro-colloidal titanium solution—Colour of 
solution: grey green in reflected light ; brown green in 
transmitted light. Ti content, about 0-6 per cent. 


LEAD 


Electro-colloidal lead solution—Colour of solution : 
grey black in reflected light ; dark brown in transmitted 
light. —Pb content, about 0-11 percene 


NICKEL 


Electro-colloidal nickel solution—Colour of solu- 
tion: black in reflected lght ; brownish green in trans- 
mitted light. Ni content, about 0-05 per cent. 


COBALT 


Electro-colloidal cobalt solution—Colour of solu- 
tion: deep black brown in reflected ight ; dark brown. 
in transmitted light. Co content, about 0:03 per cent. 


GOMMERCIAT ‘COLLOIDS 175 


CADMIUM 


Electro-colloidal cadmium  solution—Colour of 
solution: grey in reflected light; dark brown in trans- 
mitted light. Cd content, about 0-03 per cent. 


IRON 1 


Colloidal iron—Dark red lamelle or red powder. 
Colour of solution: red. Fe content, about 12-13 per 
Cent. 

Electro-colloidal iron solution—Colour of solution : 
black in reflected light; dark reddish brown in trans- 
mitted light. Fe content, 0-5 per cent. 


CHROMIUM 


Electro-colloidal chromium —Colour of solution: 
reddish grey in reflected light ; dark reddish brown in 
transmitted light. 


MANGANESE 


Colloidal manganese—Black glistening lamelle or 
grey powder. Colour of solution: dark red brown in 
reflected light; light grey in transmitted light. Mn 
content, about 12 per cent. 


MOLYBDENUM 


Electro-colloidal molybdenum—Colour of solution : 
black brown in reflected light ; reddish brown in trans- 
mitted light. Mo content, about 0-04 per cent. 


1 Compare Note 1, p. 171. 


176 PRACTICAL COLLOID -GHEM iS. 


TUNGSTEN 


Electro-colloidal tungsten—Colour of solution: 
black in reflected ight ; dark red brown in transmitted 
light. Wo content, about 0-033 per cent. 


URANIUM 


Electro-colloidal uranium—Colour of solution: 
grey black in reflected ight ; dark brown in transmitted 
light.. U content, about o:I per-cent: 


SULPHUR 


Colloidal sulphur—Grey white powder. Colour of 
solution: milky white in reflected light; bluish in 
transmitted light. S content, about 75 per cent. 

Colloidal sulphur used for injections—Grey white 
lamellz. Colour of solution: milky in reflected light ; 
reddish blue in transmitted light. S content, about 
Osperecent. 


SELENIUM 


Colloidal selenium—Dark reddish brown heavy 
lamella. Colour of solution : brick red, turbid in reflected 
light ; blood red in transmitted light. Se content, about 
Sen pelecciite 

Electro-colloidal selenium—Colour of solution: 
brick red in reflected light ; blood red in transmitted 
light. Se content, about o-or per cent. 


CARBON 


Colloidal graphite with tannin as a protective colloid 
or with mineral oil as a dispersion medium. Electro- 
colloidal carbon is a brownish black liquid. 


COMMERCIAL COLLOIDS ies 


COLLOIDAL COMPOUNDS 
MERCURIC SULPHIDE 


Colloidal mercuric sulphide—Glistening lamelle. 
Colour of solution: black in reflected light; brown 
black in transmitted light. HgS content, about 65 per 
cent. 


ANTIMONY TRISULPHIDE 


Colloidal antimony trisulphide—Red brown to 
grey green iridescent glistening lamelle. Colour of 
solution: green and red in reflected light ; blood red in 
transmitted light. Sb.S; content, about 75-77 per cent. 


ARSENIC TRISULPHIDE 


Colloidal arsenic trisulphide—Yellowish brown 
lamellze. Colour of solution: bright yellow in reflected 
light ; dark yellow in transmitted light. As,.S,; content, 
about 66 per cent. 


SILVER SULPHIDE 


Colloidal silver sulphide—Black and yellow lamelle. 
Colour of solution: grey black in reflected light ; brown 
black in transmitted light. Ag.S content, about 35 per 
cent: 


ZINC SULPHIDE 


Colloidal zinc sulphide—Brown glistening lamelle. 
Colour of solution: yellowish grey in reflected light ; 
brownish yellow in transmitted light. ZnS _ content, 
about 20 per cent. 


I2 


L7o PRACTICAL COLLOID  CHEMISiis 


SILVER CHLORIDE 


Colloidal silver chloride—Grey white glistening 
lamellz. Colour of solution: milky white in reflected 
light ; light brown in transmitted hght. AgCl content, 
about 77 per cent. 


SILVER BROMIDE 


Colloidal silver bromide— Yellow glistening lamelle. 
Colour of solution: grey yellow in reflected light ; 
reddish brown in transmitted light. AgBr content, 
ADOULZO9 Percent. 


SILVER IODIDE 


Colloidal silver iodide—Yellow lamellz. Colour of 
solution: milky yellow in reflected light ; reddish yellow 
in transmitted light. Ag content, 31-7) penscemiee 
content, about 37-3 per cent. 


MERCUROUS CHLORIDE (CALOMEL) 


Colloidal mercurous’ chloride—Greyish yellow 
powder. Colour of solution: milky grey in reflected 
light ; brownish in transmitted light. Hg.Cl, content, 
about 75 per cent. 


MERCUROUS BROMIDE 


Colloidal mercurous bromide—Yellow brown 
lamellae. Colour of solution: milky grey in reflected 
light ; brownish yellow in transmitted light. Hg,.Br, 
content, about 80 per cent. 


COMMERCIAL COLLOIDS 179 


MERCUROUS IODIDE 


Colloidal mercurous iodide—Yellow brown lamelle. 
Colour of solution: intense yellow in reflected light ; 
orange yellow in transmitted light. Hg.1, content, about 
87-88 per cent. 


FERRIC IODIDE 


Colloidal ferric iodide—Colour of solution: black 
in reflected light; red brown in transmitted light. 
Fe.], content, about o-5 per cent. 


SILVER CHROMATE 


Colloidal silver chromate—RKeddish black glisten- 
ing lamelle. Colour of solution: brick red. Ag.Cr,O, 
Conte, 2 D0uUL 70 per cent. 


MERCUROUS CHROMATE 


Colloidal mercurous chromate—Black, faintly 
glistening lamelle. Colour of solution: grey green in 
reflected light ; brown in transmitted light. Hg,CrO, 
content, about 64 per cent. 


FERRIC ARSENITE 


Colloidal ferric arsenite—Ruby red, glistening 
lamelle. Colour of solution: red in reflected light ; 
orange red in transmitted light. Fe,O, content, about 
30-6 per cent.; As,O;, about 35-39 per cent. 


180 PRACTICAL COLLOID CHEMISE Tiss 


MERCURIC SALICYLATE 


Colloidal mercuric salicylate—Grey yellow glisten- 
ing lamelle. Colour of solution: grey. Mercuric salicy- 
late content, about 60 per cent. 


FERRIC HYDROXIDE 


The pharmaceutical dialysed ferric oxide in concen- 
trations of 5 and 10 per cent. usually contains consider- 
able amounts of chloride. Colour of solution: reddish 
black in reflected light ; red in transmitted light. Fe,O; 
content, about 0-55 per cent. 


ALUMINIUM HYDROXIDE 


Colloidal aluminium hydroxide—Colour of solu- 
tion: turbid to bluish. Al,O,; content, about I per cent. 
An interesting gel of aluminium is the so-called “ native ” 
alumina, according to H. Wislicenus. 


SILicic ACID 


Very pure, neutral to litmus, silicic acid is produced 
commercially, such as the 2-6 per cent water-white solution 
used as a toxin adsorbent. The gel “‘ osmosil”’ is a pre- 
paration which has a definite solubility in cold water. 
Another preparation is W. A. Patrick’s silica gel. Colour 
of solution : aclear liquid. SiO, content, about 2 per cent. 


ZINC OXIDE 


Colloidal zinc oxide—Colour of solution: grey 
yellow in reflected light ; brownish yellow in transmitted 
hght. ZnO content, 0-66) pemeemm 


COMMERCIAL COLLOIDS 181 


MANGANESE PEROXIDE 


Colloidal manganese peroxide—Black glistening 
lamellz. Colour in solution: dark brown in reflected 
light ; black in transmitted light. MnO, content, about 
a0-per cent. 

Colloidal manganese peroxide solution—Colour 
of solution : black in reflected light ; dark reddish brown 
in transmitted light. MnO, content, 2 per cent. solution 
of a 50 per cent. colloidal MnO,. 


Peeters Ce COLLOIDS WITH SOLID . DISPERSION 
MEDIA 


Gold ruby-glass—tThis is almost colourless or faint 
yellow. The gold cannot be recognized ultramicroscopic- 
ally and therefore exists in a molecular disperse state. 
Other preparations are red to violet, blue by transmitted 
light and yellowish brown by reflected ight. The latter 
is strongly turbid, containing aggregated gold. 

Copper glass—Colloidal metallic copper is the colour- 
ing component according to R. Zsigmondy (Kolloidchemie, 
2nd Edition, p. 109). 

Silver glass—Yellow, red, violet, greenish, etc., 
colours, corresponding to increasing size of particle of the 
colloidal silver. 

Selenium glass—Yellow, red, violet, etc., colours, 
corresponding to the size of the particles. 

Colloidal colour media are present in other coloured 
glasses such as calcium fluoride in milk glass, chromium 
and iron compounds in green and violet glasses. 

Colloidal sodium in rock salt—The coloration of 
blue rock salt is in all probability due to colloidal metallic 
sodium. Blue rock salt is prepared synthetically by 
heating colourless rock salt with metallic sodium. 





182 PRACTICAL COLLOID CHEMISTRY 


B. ORGANIC COMMERCIAL COLLOIDS 


The great abundance of organic gels may be brought 
into colloidal solution by treatment with a suitable dis- 
persing medium. 

Albuminous bodies and related compounds— 
Glue, gelatin, isinglass, dried egg and serum albumin, 
hemoglobin, casein, plant albumins, such as crystalline 
edestine wet. 

Carbohydrates—Agar (d-galactose), starch, gum 
arabic, cherry gum, tragacanth, vegetable glues, such as 
carrageen (Irish moss), Iceland moss, quince seed glue, 
etc. Soluble starch and dextrin form transition solutions 
between colloid and molecular disperse systems. 

Soaps are colloidal in aqueous solutions and molecular 
disperse in dilute alcoholic solutions. Rubber is colloidal 
as a gel in benzene solution. Cellulose and its deriva- 
tives are colloidal as collodion ; as viscose, which is an alka- 
line cellulose plus CS, in water ; as filter paper in a solu- 
tion of copper ammonium hydroxide and concentrated 
ZnCl, ; and as celluloid, which is a solid solution of cam- 
phor and cellulose derivatives. Resins and resin 
soaps are colloidal in mineral oils, etc. Tannin in 
water forms a colloid transition system. 

Dyes—Typical colloid dyes in aqueous solution are: 
night blue, diamine blue, immedial blue, aniline blue, 
indigo, indulin, Congo red, benzopurpurin. 

Transition systems are Congo rubin and azoblue. 

Molecular disperse dyes in water are: methyl violet, — 
acid fuchsine, safranine, methylene blue, brilliant green, 
etc. (Sees Expt. 453) 

Chlorophyll is colloidal in aqueous solution. 

Colloidal indigo—Black particles. Colour of solu- 
tion: blue black in reflected light ; indigo blue in trans- 
mitted light. Indigo content, about 50 per cent. 


COMMERCIAL COLLOIDS 183 


Colloidal cholesterol—Amber yellow _ lamelle. 
Colour of solution: milky in reflected light ; reddish in 
transmitted light. Cholesterol content, about 20 per cent. 

Colloidal. phenolphthalein— Brownish yellow, 
glistening lamelle. Colour of solution: milky white in 
reflected light ; reddish in transmitted light. Phenol- 
phthalein content, 50 per cent. 

Colloidal tar—Dark brown glistening lamellz. 
Colour of solution : sooty grey in reflected light ; reddish 
grey in transmitted light. Tar content, about 20 per 
cent. 


DISPERSOIDS OF VARYING DEGREES OF DISPER- 
| SION 


SULPHUR 


. Large sulphur crystals. 
. Roll sulphur, microcrystalline. 
. Sulphur flowers, microscopic sulphur globules. 

4. Milk of sulphur is in a transitional state between 
coarsely disperse and colloidal sulphur. The aqueous 
suspension partially passes through an ordinary filter 
paper. 

5. Aqueous colloidal sulphur, prepared according to 
Expt. 11, or the commercial preparations. 

6. Dissolution of sulphur in paraffin oil, partially 
colloidal, according to J. Amann. 

7. Molecularly disperse sulphur solution in C5,, 


OW N H 


SODIUM CHLORIDE 


1. Large rock salt crystals. 
2. Crystalline common salt. 
3. Ground table salt. 


184 PRACTICAL COLLOID CHEMI ii. 


4. Sodium chloride gel. (Expt. 27.) 

5. Sodium chloride-benzene sol. (Expt. 26.) 

6. Molecular disperse aqueous sodium chloride solution. 

Another colloid series consists of a variously disperse 
gold ruby-glass in the three states described in a preceding 
paragraph: (1) Colourless to bright yellow when mole- 
cularly disperse ; (2) red to violet when colloidal; (3) 
blue and turbid with yellow brown colorations when 
coarsely disperse. 

Steel is a solid dispersoid, in which numerous structural 
constituents, such as the pure iron or ferrite, the iron 
carbide or troostite, the carbon or temper carbon, arein a 
state of colloidal dispersion. A coarsely disperse as well 
as a molecularly disperse state of the same constituents 
is found in other iron alloys. Colloidal carbon occurs 
besides the coarsely disperse graphite as the molecularly 
disperse hardening carbon. Specimens of iron of various 
grain sizes are likewise suitable for demonstration of a 
colloid series possessing different degrees of dispersion. 


DISPERSOID SERIES ACCORDING TO THEIR PHYSICAL 
STATE 


The following substances illustrate separate classes of 
disperse systems, the dispersion medium being given first. 

(a) Liquid-solid 1—Aqueous suspensoids of quartz, 
animal charcoal, kaolin, etc., are coarsely disperse. 
Colloidal gold, silver, etc., are suspensoids. Aqueous 
sodium chloride solution is a molecular disperse system.? 

(0) Liquid -liquid—Coarse emulsions of oil in water, 
such as commercial cod-liver oil emulsion, are coarsely 
disperse : 

1 The dispersion medium is always given first. 


2 It should be pointed out that the concept of degree of aggrega- 
tion no longer holds when the systems are molecularly disperse. 


COMMERCIAL COLLOIDS 185 


Non-hydrated emulsoids. Colloidal emulsions of 
mineral oil in water, prepared in Expt. 1, or colloidal 
sulphur, prepared in Expt. IT. 

Hydrated emulsoids. Aqueous solutions of gelatin, 
starch pastes, benzol-rubber solutions, collodion solutions, 
eit. 

Solutions of alcohol in water are molecular disperse 
systems.! 

(c) Liquid-gas—Foams, prepared by shaking soaps 
or saponin solutions, albumins, etc., are coarsely disperse. 
Colloidal foams, as yet little investigated, are seen as 
critical phenomena during the liquefaction of gases when 
the opalescence in the fluid phase occurs. Carbon dioxide- 
water is a molecular disperse system.1 

(ad) Solid-solid—Coagulated gold ruby-glass, metallic 
alloys, minerals such as granite, are coarsely disperse. 
Glass with colloidal colouring materials, steel, blue rock 
salt, smoky quartz and other coloured minerals, are 
colloids. Solid solutions, such as mixed crystals, alum, 
ammonium chloride and ferric chloride, etc., are mole- 
cular disperse systems. 

(e) Solid-liquid—Minerals with microscopic liquid 
inclusions, such as milky quartz, crystals with occlusions 
of mother liquor or water, are coarsely disperse colloids. 
Solid systems with colloidal liquid occlusions are as yet 
unknown. The water contained within zeolites may be 
removed without affecting their form, probably because it 
is in a highly disperse state, existing both as colloidal 
drops and as a continuous phase. Crystals contain water 
of crystallization in molecular state. 

(f) Solid-gas—Lava, meerschaum, pumice are coarse- 
ly disperse systems. Colloids of this nature have not yet 
been studied. Solutions of gases in solid substances, such 
as hydrogen in palladium, are molecular disperse systems. 


4 See previous note. 


186 PRACTICAL COLLOID CHEMiSi 


(g) Gas-solid—Smoke, such as soot, produced by 
burning benzene in a spirit lamp, or ammonium chloride 
fumes, produced by pouring together a few drops of con- 
centrated HCl and NH,OH into an empty litre flask. 
The degree of dispersion of such systems is variable. 
The combustion products of a faintly luminous Bunsen 
flame are colloidally disperse (H. Senftleben). 

(h) Gas-liquid—Liquid fogs, such as water vapour, 
clouds, etc., or fuming HCl, are examples of typical cloud 
formations. 


DISPERSE SYSTEMS 





Coarse dispersion. Colloidal. | Molecular dispersion. 





Increasing degree of dispersion 
| 


i 


Particle sizes | 


Y 

Oo'lu to Tuy 
Particles larger than | pass through filter | Particles smaller 
o-Iu, do not pass| paper, are held by | than IML, pass 


through filter paper, 
cans = be.) observed 
microscopically, not 
diffusible and non- 
dialysable. 


Dispersion————_~>_ <- 


Coagulation <——_—_ 





an ultrafilter, cannot 
be represented mi- 
croscopically, may 
be recognized micro- 
scopically sometimes, 
not diffused and dia- 
lysable or only very 
slowly. 











through both ordin- 
ary filter paper and 
ultrafilter, cannot 
be recognized ultra- . 
microscopically, dif- 
fuse and dialyse with 
remarkable rapidity. 


Condensation 
—> Dissolution 


Pas 
DISPERSOID ANALYSIS 


FREOUENT question is whether an unknown 
system has colloidal properties. The colloidal 


procedures outlined in this manual may be used 
to answer such questions. The following table gives a 
systematic scheme of analyses : 


A. GENERAL DETERMINATION OF DEGREES 
OF DISPERSION 


I. CHEMICAL ANALYSIS OF A HOMOGENEOUS 
SUBSTANCE 
1. According to Expts. 77, 78, and 86, homo- 


geneous appearing liquids (unless hyloteopic | Ree 
ally convertible) possess definite boiling and > 





; liquids. 
freezing temperatures, normal molecular surface meh: 
tensions, etc. 

2. Physical mixtures of materials of similar 
analytical composition but of different Iso- 
physico-chemical properties, such as melting| dispersoids 


point, boiling point, density, solubility, etc. -f eventually 
mixtures of isomers, polymers, allotropic sub-| isocolloids. 
stances and strongly associated liquids, etc., 


187 


188 PRACTICAL COLLOID CHEMIST 


II. CHEMICAL ANALYSIS OF HETEROGENEOUS 
SUBSTANCES 


Experiments of hylotropic transformations, such as 
vaporization, distillation, freezing, give two or more 
constituents of different chemical composition. 


geneous, according to Expts. 77, 78 and 86.} Molecular 
Rapid diffusibility, according to Expt. 48.) disperse 
Rapid dialysis, according to Expts. 52, 53, or| solutions. 
24: 

2. Fluids appearing heterogeneous optically upon 
microscopic and ultramicroscopic examination. 


I. Substances appearing optically nas 


(a) Macroscopic and microscopic hetero- 


geneity ; separation of components by Meas 
; dispersions 
ordinary filtration or by spontaneous sett- ; 
Meek (Suspensions 
ling, etc. Separation into two layers by eon 
moderate centrifuging ; spontaneous separ- isons 
ation (usually redispersable. ) ee 
(>) Macroscopic, often turbid, opalescent 
(Expt. 92) ; positive Tyndall cone (Expts. 77 Colloidal 
and 78), for differentiation of fluorescence). ”s 
solutions. 


Slow diffusion (Expt. 48); non-dialysable, 
according to Expts. 52, 53, or 54. 


B. SPECIAL COLLOIDRAN 


1. Viscosity not essentially greater than 
that of the dispersion medium ; easily 
coagulated by electrolytes (Expts. 153-} Suspensoids. 
158) ; spontaneously ultrafiltered spt 


57). 


Pisce RSOID. ANALYSIS 
2. Viscosity greater than that of the dis- 


persing medium; more difficult to coagu 
\ 
\ 


late by neutral salts ; decomposable by ultra- 


189 


Non- 
hydrated 


filtration (Expt. 57) ; resolvable eal emulsoids. 


scopically. 


3. Viscosity essentially greater than the 
dispersing medium, especially at small con- 
centrations ; greater temperature coefficient 
of viscosity (Expt. 70). Difficult to coagu- 
late by neutral salts (Expt. 162). Dzisper- 
sion medium and disperse phase not com- 
pletely separable by spontaneous ultrafiltra- 
tion. Separate particles not recognizable 
ultramicroscopically, but only by Tyndall 
cone. 


Hydrated 
emulsoids. 


TABLE OF NORMAL SOLUTIONS 


HE concentrations in grams per litre refer to the 
hydrated salts of the composition given. A 
molar solution of BaCl, contains, for example, 

208:3 g. of anhydrous salt per litre. Since the usual 
commercial preparation has two mols of water of crystal- 
lization, the following table gives 244-3 g. dissolved in one 
litre of water, etc. The bracketed numbers denote that 
the molar or normal solution cannot be prepared on 
account of small solubility, which is given in column 3. 
The data of saturated concentrations are given for 
15° C., if not otherwise stated. The concentration data 
in column 3 also refer to the hydrated salts. 


























MSine Saturated 
olar Normal solution 
Substance. solution solution erams per 
ey oie 100 grams 
per litre. per litre. of Sblmtians 
ARNO. = o).- Eta 169°9 169°9 64:9 
AICI, - C2 ent sy ae 133°5 44°5 4I°I 
AL (DOE Olt. eget 666-7 TL TL 50°4 
BaCl. 25. O sere 244°3 1222 31°0 
CaCl iene. (cate Rone Tilo 53°5 -41°0 
Ca@i 661,07 anne 219°1 I109°6 80°90 
CasOfi! 1 sagas (136-1 68-T) 0°20 
CasO 2H OR es (72:2 86:1) = 0°25 
CdSO7* HO aan 256°5 1283 92:6 
CoC) tg eee 134°5 6752 43°0 
CusOr 5 Osan ae 249°7 124'9 25°3 
FieCle ahve cere aae 198°8 99°4 63°1 
HeChs ete Ue aes 162°2 54°1 46°4 
HéeGl.- OH (seen 270°3 gorl 77°3 
FeSO (77 rsO) ee ee 278:0 139°0 35°4 
Urea (CO(NH 2). i252: 60°1 — 42:0 


190 








feoee OF NORMAL SOLUTIONS I9I 
Molaz | Normal | Saturated 
Srhstance: solution | solution grams per 
grams per grams per 00 grams 
litre. litre: Breather 
Citric acid 
UcIAG4 SORA! a] 210°1 70:0 64°8 
HgCl, (271°5 135'8) 6°54 
Hg(CN), va ar (252°6 126:3) 74 
KAI(SO,),.12H,O (474°5 1188) 8°75 
KBr ee IIg:0 II9g:0 38-9 
moN. 651 65°1 a 
KCNS . 97°2 97°2 67°5 
Tichol) a's 138:2 69°1 52°5 
KCl 74°6 74:6 24°4 
KCIO, . (122-6 122°6) 5°79 
K,-citrate 
[K,C,H,O,.H,O] 324°3 108-1 64°7 (30°) 
K iFe(CN)e aH 0. (422:6) 105°7 20°6 
BSL eins 166-0 166-0 58°4 
KNO, TOI-I LOtt 20°7 
K.SO,. (174°3) 87°1 9°25 
Pinos ~ I109°9 55:0 25°7 
MgCl, . . 95'2 47°6 55a 
MgCl,.6H,O 203°3 IOI‘7 75° 
MgsO,.7H,O . 246:5 123°2 51:0 
NH,CNS . 76°1 76° 60°7 
NET CI . 53°5 53°5 26:0 
(NE ,50% 1321 66:1 42°6 
ELS bee 58°5 58°5 26°4 
Na-salicylate 
[NaC,H,;O3] 160:0 160-0 52-0 (at 20°) 
Na,SO, eee 142°1 71-0 BIT 
Na,5,03.5H,O 248-0 124°0 62-0 
Na,5O,.10H,O 322°2 I6I°I 26°5 
Oxalic acid 
feet OO 2ri.0| (126-06) 63°03 10+2 
Pb-acetate.3H,O . 379°3 189°7 ca. 30 
PbCl, : (2 Fond 139°I) 09 
Pb(NOs;)» 331-2 1656 e353 
J Ne) eg d's We Oe 28755 143°8 60-0 





























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